Sansui BA-3000 power amp loses power after a few minutes

[visualdave]

I have a Sansui BA-3000 power amp and after it's on for 5 to 20 minutes, it loses most of the power in one channel and then the other. If I unplug the source and then plug it back in (resulting in a loud burst of connection sound), or if I turn it off for a while, the channels are fine again for a while and then it happens again.

Ideas on what I'm looking at here?