Whenever I see a variety of electrical problems that change, or strange happenings, I think "Bad grounds" When things lose their grounds, they backfeed through other stuff and can cause all kinds of strange stuff.

The easiest way to check grounds is to double them up or, in other words, add another temporary ground.

If your extra ground solves the problem, or changes it, you have found the bad ground. Or perhaps one of many!

One way to add electric reliability to an old brick is to clean all the connections and put them back together with dielectric grease.
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'96 855R,'64 PV544 driver, '67 P1800 basket case, '95 855, '95 854, the first three are mine, heh, heh, 415,000 miles put on 9 bricks