M4 and EMI (small) perturbations

[Duke]

And people are just going to keep spouting the same "clean the contacts, get a higher rated PSU" line that prevented me from solving the problem for literal years.

I know you had a special case (also this is early M4 which wanted close to 5V, his is latest version), but this is what is the normal issues and solves it for 98% having problems, I am not saying there couldn't be cases where this is not so.
I could mention a handful of users having issues that were solved replacing the Z80 chip, but when this is 5 out of 1200, it is not the first thing to suggest.
So as a rule thumb, iron out the edge connector first which is the usual problem if there is one.

[pelrun]

Yeah, you don't want one of those CMOS z80s, that's for sure.

Interesting that replacing the regulator on the M4 improved it's reliability; this is the first I'd heard of that particular change.