I have had "static crashes" due to dry electrolytics. If you have a scope,
you might watching the dc lines to see if you can observe any of the
"crashes" there. I would also replace all of the paper caps.

It could be a number of things, such as a poor ground caused by oxidation or
even a noisy resistor. A scope should be able to help you isolate the
general area.

Note that if it is a bad electrolytic, often paralleling a good one will not
eliminate the crashes. The bad one needs to be removed from the circuit.

Hang in there. We all go through this at one point. I remember my friend
had a microphonic mica cap iinside an if transformer of his NC-183D. He
worked on it every night for 6 months until he found it.

73, Colin K7FM