Cary SLI-80: Thunk in Right Channel, then Dead

I recently bought a Cary SLI-80 on A'gon and been having a fine time with it tube-rolling and auditioning CD players to match it to. But, just tonight, I heard a "thunk" or lower mid-range "pop" and the right channel went dead. I tested this with headphones, and it's definitely the amp and not the speakers, as there was no right channel in the headphones either.

However, after a few minutes checking all the wires and connections on the back of the amp and on the speaker, I fired the amp up again. The right channel was still silent for a few minutes then miraculously came on all of a sudden. It played fine this way for about 35 minutes, then the same "thunk" or lower midrange "pop" came through the right channel, whereupon the channel again went silent.

I shut the amp down, as I didn't know what else to do except write the former owner and a few audiophile friends whether they'd had any experience with this phenomenon.

Finally, I turned the amp on again for a third time, and the right channel played fine from start-up. I don't know how long it will do so, however, and I'm worried I might damage something further if I kept playing it.

It seems to me something is "shorting" from inside the amp, but all the tubes still glow. I assume the fuse would blow on a full-scale short. Could it be something as simple as a loose wire or poor soldering job?

How can I address this? Advice anyone?
Tommorrow I would call Cary in the AM. Phenomenal customer service and can troubleshoot may things over the phone with you.

Thanks for your suggestion. I called and spoke to Gerald Walsh at Cary, who suggested a method of tube-switching that established quickly I had a faulty pre-driver tube (one of an NOS pair I'd just gotten) when the thunk sound traveled to the left channel when I switched the tube from the right to the left side of the amp. I subsequently learned from Gerald that a faulty pre-driver could send too much current to the powertube, which would cause the red flash I saw in the far right EL34 in conjunction with the thunk sound and silence that followed. It still is a mystery why the channel went dead, as Gerald said there is no single-channel auto-mute function but that the muting should work on both channels simultaneously. There was some concern these instances might be causing damage to the capacitors or resistors, but, after changing pre-drivers this morning, all seems functioning normally. Thanks for your suggestion, which eventually turned up the cause of the problems.