Opinions re: debugging/repairing Cambridge Audio 540A amp stage

Hey! My old Cambridge Audio 540A has seen better days. It's been acting up recently. I'm interested in fixing it up myself if I can, but I'm looking for opinions on where to start.

Symptoms: speaker output on *any* channel (L/R, A and B) can drop instantly from the correct level to partial (including zero output). It does not happen consistently: on some audio streams, it seems more stable, but I have no way to reproduce reliably. The drop is instantaneous, not a slow shift. I can reset the audio levels if I switch input selector and back, until it happens again. If I do nothing, sometimes the audio will click back into place on its own.

This can happen on any input. Multimeter suggests that the voltage on the output drops when the audio drops; I don't suspect an issue with the speakers, but I don't have spares to test with. Headphone output is always perfect. My suspicion has been the amp stage in the 540A.

This is not a new unit, and so I popped the top off. My first suspicion was the big capacitors, because of age and the gunk they'd deposited on the board: https://i.imgur.com/u0Qnj8O.jpg  So I've cleaned those and replaced them. The problem persists, but I didn't have replacements to hand for the smaller capacitors.

I'm feeling my way around as I go; I have a copy of the service manual so I can decipher things, but I'm looking for good approaches to debugging these units to home in on the problem.

Note, obviously I can replace this unit easily. But if I can fix it, I'll feel happier, and I'm also happy to get my hands dirty. I'd appreciate pointers from folks on what to attack!