While driving vehicle all of a sudden lost acceleration power?

This all started as I was driving down the highway, my vehicle all of a sudden lost acceleration power and the little wrench emblem popped up on the dash. The vehicle did not turn off, however I was not able to accelerate and the vehicle stayed in an idle state while holding 700 rpm's fairly well. I had the vehicle taken to the mechanic and he said it was the throttle body. He replaced it with a used one to test it and the vehicle had the same issue except now the vehicle would shut off after a while. He told me that it was the ECM and insisted I take the vehicle because he does not work on electrical problems. The car does not rattle, shake, or do anything abnormal except the problems stated. I have not replaced any plugs or fuses. As I was getting ready to tow the vehicle the battery was completely dead since the vehicle sat in his shop for a few week (he waited so long to tell me these problems let alone work on my car). As I jumped the car to put it in neutral, I figured to give it a shot by putting it in drive. The vehicle was drivable for about 15 minutes when it began to showcase the same problems, a complete loss of acceleration power and staying in a idle state while holding 700 rpms. When the vehicle is in park or neutral and the gas is pressed down, it will sometimes but not high, maybe around 1500 - 2500 rpm's. Are these signs of a faulty ECM? I can’t pinpoint this issue, another outside source even stated that it could be a transmission solenoid since the vehicle was able to drive for a bit and all of a sudden stopped accelerating. I was not able to receive codes from the mechanic because he stated that he had forgotten and could not get to my vehicle at that moment for another diagnostic run. I have never experienced such a terrible mechanic by being put on the back burner. I would greatly appreciate the time and knowledge about this problem and hope to take care of this myself. Thank you for your time.