Vintage Synth Repair

I have recently repaired a random reboot concern on my
JX-8P by replacing the 7805 voltage regulator, and resoldering
any possible cold solder joints in the power supply area such as the rectifier joints.

A new random type issue has developed
and I think it is a wire and or joint in the
main board.
Sometimes you can hear DCO2 cutting in and out
while playing, and if you tap the keyboard case
around the JX-8P logo, you can hear the DCO2 sound
kick in and out, responding to my fingers tapping.

It seems this would be a loose connection either on a solder
joint, or one of the connectors,
but does any technical people have any idea
of where on the main board I should be looking,
as this is a microphonic style random issue.

Thanks

Mark Pigott
Sarnia, Ontario