Sony STR-KM77

Brought this was supposedly working (luckily the seller was the good sort) got some of my money back. So when i first powered it up it went into protect mode then shut down so unplugged it, plugged it in them turned it on and it started clicking like the main replay was going on off on off it must've done it like 7 times before it shut itself down so i unplugged it again, then curiosity got the best of me so i plugged it in again so it turned on and stayed on but it froze up you couldn't change the input or volume but the volume dials lcd would flash like they do when there changed so that was just what happened for a few cycles of turning it on and off the i moved it to show the previous owner and while we were messing around it had been left on for like 10 mins by this stage and it started working as normal apart from any sound coming out the speakers, anyway after playing around with different setting it started inconsistently pushing sound out the speakers so this is where i am at now.

I will turn it on sound works fine for 15 seconds gets less and less consistant then nothing untill you turn it off again, then the cycle repeats once you leave it for a bit

Any idea what area to look at specifically?