Basically the more voltage you put into something, the more heat has to be given off to drop it down. So I began to notice this more with my old Sega Model 1 Genesis. Been fine for years, and then I got a Sega CD model 1 to add to it. I noticed one day when I left the sega CD bios screen just up and running that after about 20min or so, the graphics started to form this snow pattern all over the screen. Odd thing was that it was only affecting certain graphics that were being generated by the Sega CD itself. This also started after I had just recapped it?! I checked everything and couldn't find any issues to cause this. Then I tested it with a model 2 Genesis and another model 1 genesis. And it didn't happen with those.
Finally tracked it down that my sega genesis itself was generating WAY more heat than the others for unknown reasons. Replacing the 7805s in it with higher amperage rated 7805s took care of the issue. Seems that the increased voltage from the PSUs was starting to burn out the original 7805s and they in turn were having to work harder to do what they needed to do and weren't providing as much current and voltage as they should be once they got to a certain point in heat. I've also swapped out the psu and use a modified gamecube supply now to power the genesis, sega CD, and the 32x from one switching power supply brick. More efficient and heat is virtually non existent on it now.
Midi Modules: MT-32 (OLD), MT-200, MT-90, SD-20