2014 Jeep Cherokee Trailhawk "Shift to P, then desired gear" Problems

My 2014 Jeep Cherokee Trailhawk has been having this issue since about a month or month and a half after purchasing it in January. I know that buying a used car comes with a risk, especially when it's only a few hundred miles off from having 150k miles on it and is 9 years old. Unfortunately, that's where a lot of the major issues start to come in, hence this post.

At first, it ran fine. No check engine light, no funny noises, no weird smells, nothing. Then it started to have a delay in acceleration; specifically on inclines of any kind. So when I would be turning onto an incline, it wouldn't get up to speed right away then it would be fine once the road flattens out. I assumed it was a transmission issue, since all transmission issues I've seen and experienced over the years would start with acceleration issues in particular.

After about 2 to 4 weeks, the check engine light came on and a burning smell came through the vents in the cabin when I used the AC and heat. I called up my local mechanic and gave him the run down of the issues I was having and asked if he could look at my transmission and see what the burning smell was. After over a month of him having the vehicle, test driving it every day with and without his code reader hooked up to it, he couldn't find what the issue was but eventually found and fixed he oil filter housing and flushed out the transmission fluid. He test drove it to make sure it ran fine, which it did for him at the time, and the small local dealership I bought this car from covered the bill and that should've fixed our problem, right? Wrong.

Not even 3 weeks later, it drops gears at three different intersections in a city 2 hours from home, and note: there's no check engine light on. It first did it on a pretty steep incline at a stop sign. The car stayed running, dash and everything else was on, the only problem was the message on my dash that read, "Shift to P, then shift into desired gear" and I couldn't move my car until I did it. So, I put my car into park, waited about 30 seconds, put my car into drive, then it was fine until a couple of stops later at a light intersection. I did the same process that time and it ran fine until my second to last stop before hitting the interstate, did the same process again and it ran fine the rest of the way home.

We call our mechanic the next day and he says his code reader doesn't throw transmission codes at him, so we have to go to the bigger dealership in town that specializes with Jeep products and has Jeep compatible parts and tools. So we do, they drive the car around and run their diagnostics to find that the transmission is fine because there aren't any codes popping up for them, but the rear driveshaft and an engine torque strut need to be replaced. The engine would move about 4in every time it was started and moved. Okay, I eat the 2k in repairs, pick the car up, and it drops gears again the day after receiving it, again, no check engine light. It dropped gears just like how it did in the city BEFORE any repairs were made. We take it to that same dealership the following Monday, have a mechanic ride with us to try and catch it in the act, and nothing happens. There's nothing they can do, even after running diagnostics again and again, and after showing us what their screen looks like when something does/doesn't pop up.

About a week or two goes by and I'm trying to catch this issue on video so the shop knows what I'm talking about since they've never experienced this problem before. During that time, my car is chugging, sputtering, delaying upon acceleration on inclines and now declines, all of it. The only way I can explain it is like it's having a hard time getting ahold of the gears it needs, but it's never dropping. I finally caught it on video three times this last Saturday, except now it's a little worse. It specifically started to get bad on Saturday after I got gas that morning, so I don't know if that could be a factor as well, since it never did that after getting gas before.

When it dropped gears I couldn't move my steering wheel, as if my power steering pump went out and it stops running altogether, but the dash, touchscreen, and lights are all on like it's in RUN, not START. No. Check. Engine. Light. That message pops up again, "Shift to P, then shift to desired gear." I do exactly that, but this time it doesn't move. The car doesn't start up, even though it's in the START position. It pops up with that message once more, I go to shift into park and my brakes are locked. So I put my car into park, turn the car off and back on, and I'm able to move it again. I pulled into a nearby parking spot and the "Battery Voltage Low" message and image pops up, but immediately goes away. I had it off for about an hour or two, then it dropped gears at a light intersection on a slight decline after I started it up again. I have to put the car into park, shut it off and turn it back on in order to move it again. The same exact thing happens the third time and I do the exact same thing to get it to move again. It made it home with just the chugging thing every couple of minutes or so, but that shouldn't even be happening.

I showed the mechanic the videos two days ago and took it to the shop this morning to have them try to catch the issue in the act. But the problem I'm now running into with this whole thing is that they can't figure out what it is until it happens while running the diagnostics at the same time. The check engine light never comes on before or after it drops gears and it isn't showing any codes when it's driven. I'm starting to think that the only reason why they haven't been able to get to the root of the issue is because it'll sit for a few days without being used so it's like the problems will reset and you have to wait awhile as well as drive it consistently in order for anything to start happening. It'll run fine for awhile then the chugging, sputtering and delay upon acceleration will get worse a day at a time until this ends up happening. This is the only way I can indicate when it'll start to drop again.

I've done some research, but it doesn't seem to get me anywhere. I'm either giving an arm and a leg to just fix something that could potentially fix the main issue, or the people having the same problems as I am have no answer either so we're all stuck with a pricey lawn ornament.

I mentioned to the mechanic I'm surrently taking it to that it could probably be the transmission module going bad, a clogged catalytic converter that just needs to be unclogged, or a pinched or corroded wire somewhere since those are the main things I saw and read up on a little. I figured explaining my situation on here could help both me and the shop out to try and dig into a few potential issues, instead of gutting and dissecting the whole damned thing. Thank you for all advice and replies in advance!