Hafler DH 500

Does that occur with all sources, or just one? How frequent are these malfunctions? Can you hear the relay clicking, with no input to the amp(ie: preamp turned off or disconnected)?

I shall have to test if the amp does this (clicking sound without input source), and also test with other source. I will keep in touch until I come back from vacation. Thank you very much! You are very helpful.

P.S. The frequency is every 2-3 songs. It goes off 2-3 seconds and goes on again.

Those tests should reveal a lot.  I'll be watching, for your further info.   Hopefully, we'll figure it out!   Have fun on your vacation.