Q: 1996 Olds Aurora OBDII P0603 PCM keep alive memory, car is in limp mode and running like it's off timing

I have a 96 Aurora, 8 banger 4.0 and it's running like it's firing on less than 8 cylinders because of an OBDII code P0603 Keep alive memory error/reset (translation, it's running in limp mode). I recently had the plugs and wires changed, oil, radiator flush, tranny filter, new battery, I dd find a loose ground wire that begins somewhere under the intake manifold cover and was bolted to the block, my mechanic must have accidentally broken it during the plug wire change. It's only about 16 gauge so it's not like a ground strap or anything, it must be a component ground, which could be throwing the code. I'm on disability so I can't just keep replacing parts, when this is clearly a connection issue. I need an honest mechanic who won't tell me to replace the ECM, as you can see, I do know my way around cars. Thanks much

My car has 118000 miles.
My car has an automatic transmission.