Randomly car stops accelerating and check engine light is on

Hello, I have some questions about my daughter’s car listed above that she bought from a junk yard about a year ago.
The former owners lowered the car so much it barely makes it over speed bumps. They also bypassed the CCRM and jumped the cycling switch, so the system wasn’t cycling right. We had the CCRM replaced, along with the cycling switch and pigtails and we repaired all of the wiring that was rigged together. The car was running a little rough but there weren’t any obvious issues with it until a few months ago when the check engine light came on and it started having a problem accelerating when leaving a stoplight, turning, or while driving on a road with an incline. We changed the oil and did all of the other maintenance we could do, and we replaced the spark plugs, but the check engine light is still on, and the acceleration issue has gotten worse. A few days ago, I pulled into a gas station at a normal speed and the car just completely stopped accelerating. The cars that turned in behind me had to slam on their brakes to avoid hitting me. My daughter lives in Phoenix, where the traffic is completely crazy all the time, so she has been walking to school and work until we can get this figured out. She had taken it to a mechanic, but they said they drove it for 30 minutes on the freeway and they found nothing wrong with it that could be causing it to lose power the way we described. My son does the maintenance and most of the repairs to all of our cars but he’s not a mechanic and he wasn’t able to figure out the problem with this one. One thing I want to mention is that the car did not have a gas cap when we bought it. There’s a fuel door that closes but the gas cap was missing, and we have no idea how long it had been like that.
My daughter absolutely loves this car, she had wanted a Mustang since she was little, so she worked and saved and was thrilled to have found this one, but I have a feeling this one is going to cost more to repair than she paid for it, (I didn’t even go into the problems with the suspension). The acceleration problem is a safety issue and needs to be resolved, I just don’t know where to start or who to take it to. If anyone has any ideas about what the cause might be, or what we should be looking for, I would appreciate any help or advice that you are willing to share.
Thank you very much!