kinda reminds me of the weird R-Y amp problem i had with my GE CTC-15 clone.
each time I THOUGHT I had it fixed, it kept coming back.
https://i.imgur.com/BSJ5KpG.jpg
normal
https://i.imgur.com/rLZv8JY.jpg
messed up
I finally got tired of trying, and got some 30 Gauge Solid Bare Copper Wire and point to point wire wrapped / reinforced every trace in the areas involved, and it has not done it now for 8 months, I guess re-soldering the PCB 4 times alone was not enough!
GE did NOT live up to their name, " good enough " :O