2000 Kia Sportage keeps having similar problems, even after 3 fixes, what's actually wrong?

This is a long explanation of the problem I’m having, but I want to give as much info on the car’s issues so that it can be properly diagnosed and fixed, which evidently the mechanics I’m taking the car to aren’t doing. (At least not for a significant length of time.) So please everyone bear with me, and if you know anything, help!

About a year and a half ago my 2000 Kia Sportage was misfiring so I took it in to get repaired, the mechanic replaced the spark plug wires, spark plugs, wire harness, and mass airflow and I think probably the coils. After this the car ran fine for about a year but during that time the Check Engine Light came on and reported a minor emissions leak (which I believe is a P0442 code) which said to replace the gas cap, which I did, but the CEL never turned off and I never purposefully reset it. Well after a year of normal performance, I started to have a problem with the car which was new to me.

When I turned the car on everything would seem fine, but hitting the accelerator would almost bring the car to a dead halt and it would jerk violently, the only way to get past this was to give the car a lot of gas, which sometimes I would do by driving (pitifully slowly and awkwardly) around the neighborhood, and sometimes I would just put the car in park and give it gas then which would result in in running normally when put back into drive. Obviously this was a fuel mixture/fuel system issue. When I got the car diagnosed at the local auto parts store the codes it gave were the old “emissions leak” code it had always had, as well as one that involved the mass airflow. Well, since at this time the mass airflow was only about a year old, I didn’t think that could be the issue so I took the car to the same mechanic that had fixed it before and explained to him what was going on.

He said that the issue was that the fuel injectors had become clogged, and so had the mass airflow. He said he ran a fuel system cleaner through the car and the fuel injectors were spitting all kinds of carbon out and he had to clean the fuel injectors 3 times. He then told me that I was to under no circumstances put fuel system cleaner in the car, or it would just junk them up again. Thankfully he didn’t charge me that much to get done. Well after this happened, the car worked with no issues, for about one week, oh and the emissions leak CEL code came back.

At this point I started to have a different problem, but one that I thought could be related, instead of the car stalling when the accelerator was depressed, the car would now misfire and sometimes stall out completely when it was idle, but oddly, this would only ever happen after driving for 15+ minutes, and it almost never happened in park. (I say “almost never” because I drove the car in this state for some time because I simply couldn’t afford repairs until I eventually had it fixed for the 3rd time which was about 3 months ago, also, sometimes I didn’t even need to drive the car for a long enough period of time to cause this symptom to happen. Over time this got worse and much more common and it even started happening when the car was in park, which eventually led me to get the car fixed a 3rd time.)

Since the mechanic who had now worked on the car twice had literally just cleaned the fuel system one week ago at this point I took it back to him and explained the new symptoms, he told me that the car had probably “learned a bad fuel mix” from the time when it was all clogged and since I had a very low tank of gas when he fixed it. He pulled out the mass airflow sensor and sprayed it down with something while I was there and reset the CEL and told me to see if that fixed the problem. For the time being it did, but both symptoms came back within 2 days.

Like I mentioned before, since I really couldn’t afford repairs and this was a problem that only happened if I needed to drive for awhile (which I rarely did) I just left it alone until it got worse. Then about 3 months ago I finally took the car in to a different mechanic this time. I explained to them everything that had happened and everything that was replaced in the past year. They diagnosed the problem as the coil kit, and replaced it. This solved all the problems I was having, except the CEL was still showing an emissions leak (to be fair, the CEL would always be off for about a day or so before it finally showed up again, and each time I would check it, it was only the P0442 code, and like I said, I replaced the gas cap so I’m almost certain that’s not the issue.) So now, I’m 3 months after my last repair, and suddenly the problem where the car won’t accelerate is back, like the fuel mixture is bad or the fuel system is clogged, and ALSO the car will stall out and jump violently when idle, even in park. It’s like both of the problems I’ve had fixed in the past year and a half have returned at the same time so I’m starting to believe that there is something else behind the scenes causing these problems.

Other symptoms I’ve noticed during this time:

The car will rev up to about 2 or even 3k RPMs when turned on for the first time in a while, it will then settle down and act normally until gas is applied, then it chokes.
After I’ve reached the point where the car starts to misfire/stall when idling, if I apply the brake and sit there, or just let the car sit in park, the RPMs fluctuate rapidly with no application of the gas pedal.
It’s an automatic transmission, and sometimes when driving, the car doesn’t switch gears when it should and requires a lot more gas to get it to switch gears, sometimes the idling/misfiring issue happens only after this has noticeably happened, other times this happens and the car doesn’t start stalling at idle.

The last time I drove the car it performed very poorly and stalled out multiple times, I absolutely can not afford a professional fix at the moment, and after 3 attempts that clearly didn’t solve the problem (at least not for any significant length of time) I am really in trouble if this isn’t easily fixable.

I’m going to get another diagnostic done as soon as I can to see if it has any codes other than the emissions code, but at this point I’m beginning to suspect that the emissions code might be part of a bigger problem that’s causing the rest of this mess.

P0422 would point me in the direction of either the converter or the O2 sensor. Check the wiring to the O2 sensor. The fact it occurs intermittently might mean a wiring issue.