I have an HK570 i with similar symptom that i have not troubleshot yet,
I just bang on it and it fixes it for a while. in my case it gets
louder on only one channel and the tone changes such that the mids are
boosted but the lows and highs are not. In my case it sounds like
there is a bad solder joint in some feedback circuit someplace where
the negative feedback is getting disconnected.

Next time it happes check the effect of all the controls, tone balance
etc modes, all that info will be helpful troubleshoting.
Mark