Buzzing output pair after 20 seconds of playback

Hi all, I have 5 speakers hooked up to this amplifier. I have problems with surround LR pair.

The problem only occurs after 5 minutes, buzzing starts to occur on loud parts of the song (for example accented notes, snare drum). Then two more minutes later it develops into a constant buzz. There is still sounds playing underneath.

After resting the unit for 1 hour it resets back to normal. Then the problem recurs 5 minutes later.

Other channels are working without a hitch and I am certain it's a problem with this part of the amplifier. Which part should I be investigating? I also have access to the service manual so can find and order new parts.

Many thanks in advance.

Thanks for the hint, I just opened the unit today and it seems like the FET is on the underside of the pcb, so I will have to continue that tomorrow.

In the mean time I had another look at the schematics. Is the circled the FET in question? If so how do I know if it's getting too hot? And if it's a small thing enough to be under the Pcb?

Thanks again oldturkey I really appreciate your answers!