"Could possibly be a bad solder joint that doesn't make contact
until something warms up and expands."
That is TRUE.
I have had the same problem but I do not realy bealive this could be
bad solder joint.
Finally I unscrew many screws and find bad joint
at R467 near zenner dide ZD404 and IC408.
Also check all resistors and zenner diode from +12V and -12V supply
which are under thermal pressure.