The volume of my speakers going down by itself and going back again in a minute or two

noromance has a point, turn off the lights a see if ALL the valves get dimmer then brighter.

Do you need to bias the valves individually, or is it auto bias?

Are the small valves doing stupid things?

Did you pull the valves and then plug them back in? Stranger thing have happened.. None of this stuff cost anything.

Getting the holy water might cost you a fiver. :-)

Regards

Sounds like the new gear to me. 
 

  Send in and get caps replaced, looked at, cleaned, ther stuff replaced.

send or drive to factory if they allow it.

shipping sucks!

 Pay the money, that is some fine gear!!

Swap out amp or preamp to determine which piece is giving you the problem. If you are not a tech, then you're going to have to send that unit in for service. Could be something simple like oldhvy mentioned, worth checking.

is a built-in test sequence occuring?

If you weren't already an old hand at this, I would have suggested going back to first principles and cleaning/checking connectors and volume pots, but I'm sure you are way beyond that.  Good luck with the gremlins!