Help Please! 47rh OD issue different than most

I know I'm going to catch a lot of heat for this and yes i have spent the last 3 days reading posts and searching forums and still can't find a concrete solution. I picked up a 95 auto trans today and am having a similar OD issue but it has a few added elements I haven't seen anyone else post about. My symptoms are check engine light will blink then go out, tach bounces around then dies, and the OD won't work and the cruise control won't work. Previous owner says it only happens in hot weather and has never happened in the winter. From what I read, thinking it could be temp sensor, it seems most people experience the issue when it's cold not in the heat? He also said sometimes even if the truck was parked in direct sunlight on a hot day it would happen immediately? I'm thinking its got to be temp or heat related, he said a diesel mechanic told him to replace the ECM.

Went on a half hour test drive yesterday and it was fine, today it was about 10-15 degrees warmer60-65F) and it happened on the way home. He's already done the alternator and CPS. Also a manual switch in the cab But OD does not engage with the switch it just feels like its not revving as high with the switch on. Sorry I'm a cummins newbie (coming from a twin turbo audi, way different animal). I got a smoking deal on the truck and bought it because i need to rip my other car apart and rebuild turbos and heads, but im worried I just walked into another nightmare. Thanks for any help in advance.