I am by no means even an intermediate level repair guy when it comes to pin boards. That said, one thing I look for first, is stable 5v and 12v. When I hear reboot, that’s what I immediately think. A simple multimeter can check both. Too low 5v and the MPU can act up. Many times the capacitors that provide a stable flow of power just wear out.
Additionally, any chance I get to reflow connectors, I do as well. Tends to fix many ghosts in the machine....
One final thing. If you have the original MPU in the machine, i hope it doesn’t still have the original battery on it, does it?
- My MPU was destroyed when I bought my machine years ago and I put in an ultimate MPU.
If you don’t have this, a great reference tool as well.
Go with jj44114 suggestions first though, much more knowledgeable on boards than I am!