JVC VR-5535x - Strange issue and fix (?)

[michalism]

Hello all, and warm season greetings.

I'm working on this old JVC amplifier, which I bought in the flea market. After some basic clean-up and checks, I stored it away as I had other projects to work on.
Last week I got it on the bench for some more thorough checks and final restoration.

Then I discovered that one channel was not working- not completely silent, but for sure there was a problem there. I noticed that when I turn the volume all the way up, it there's some signs of life but still not proper playback. I'm pretty sure this issue was not there last time I checked, but since I have kept no notes, I can't be sure.

I checked and replaced all electrolytic caps on the main amplifier board, and then set-off to do some signal tracing and voltage checks.

To my surprise, when the signal tracer probe touched point 602 on the schematic, there was a loud pop, and the speaker started playing normally. It played for a few seconds then got silent again. Touched the probe again, same thing. Even when touched with a screwdriver, it would start playing for a few seconds or minutes then get silent again.
After a couple of days testing and scratching my head, the amplifier is playing fine now! I had let it play two or three times for a couple of hours each time and it is stable. I have even tested it with an oscilloscope (1kHz tone, 120mV p-p on AUX input.) and seems perfect!
But I'm still worried that this may come back later on unless I understand what was the problem and why it is fixed.

I want to get to the bottom of this not only to learn, but also because this will soon be my main amplifier, and I want to make sure I leave no stone unturned.
 
I'm attaching the schematic of the unit, the service manual exceeds the 4Mb limit, but I can try to cut it in pieces if it helps.
Any help would be much appreciated!!
Thank you very much in advance!
Michalis