Vintage Synth Repair

I have a Fostex Model 600 power amp.
A while ago one of the channels started to quietly pop and click.
I pulled the 1/4inch jack (from my mixer) in and out and it cleared up (the plug socket is not switched) so I thought it was a dirty contact.
The problem returned and was fixed several times.

Now the pops and clicks are intermittent and the replugging has no effect.

It doesn't seem thermal because if it is left alone with just the quiescent current flowing though it, it can start noisy then it can be quiet for and hour and the pops just start up again.
The DC measurements are close to the cct diagram.

Before I start replacing all the caps, should I be looking at some other components?

Thanks

Royce