Audio Research SP3 loses volume

My SP3 sporadically after getting warm suddenly has very low volume output. Sometimes it comes back by itself.

It does not seem to be a problem of the amplifier stages, because I already replaced all tubes, no change.

I think it is a matter of the power supply.  Whenever I tick on the transistor with the meter probe, the sound comes back for a while.

Does anyone have experience with a similar case?

Thanks in advance
Ralph