hey im new here I have a 99 grand prix 3.8 with a automatic tranny that is stuck in neutral right now. in april my car started randomly shifting itself into neutral while I was driving or if I was at a light for too long. If I restarted my car it would then shift and I would be able to move again. just yesterday when I went to go to work the car despite my best attempts and all my tricks the car would not budge.
I took the car to a tranny shop and of course they couldn't tell me what was wrong with the car exactly. it was my hopes to fix whatever the issue was myself with a few local mechanics help. I think its a solenoid but im not sure. can anyone help me out?

also If I do have to replace the tranny I have a couple of friends who have connections to get a used tranny under 100k miles and a tranny from a 01 gtp. my concern is someone had told me that "the computers have to match up or be in the right place" does anyone know what the hell this means
thanks.