>> The power supply idea is a good point; I should find an accurate
>> voltmeter and measure the supply rails.
> the "accurate voltmeter" you are looking for is a scope!

True.  But I don't have a really good 'scope.  The only 'scope I have,
while mostly a good piece of work, has a severely washed-out final CRT
face, to the point where it's unusable if the lights are on.  It's also
got rotated some 5°-10° somehow.

I keep meaning to dig into it in enough detail to figure out if I can
replace the CRT with another, different, one I have , or if I'd need an
identical replacement.

> when DC power is/gets noisy/spiky/ripply this does not necessary mean
> that the average voltage (the value all DC voltmeter show you) goes
> "out of band"

True enough.  But (a) it can't _hurt_ to check the average DC voltage,
and (b) since everything works when kept cool I am inclined to doubt
that noisy power is the root of the problem.

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