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

Without any further information I'd say it was demon possession, it could just a curse. Two Hail Marys and 5 Our Fathers for the curse, and get some Holy Water for the possession.. That should cover everything..

Then look at the sticking remote, and change batteries and clean the contacts if need be. Just a thought..

I'll put in a good word before sundown.. It is a day of rest.. :-)

Your new to you 20 year old pre and power amp might need a tech check.

I had the same thing happen. Then I realized the dog was stepping on the remote. All 7 pounds of him. 

Overheating? Turn off lights and look at tubes for red plating.

I'm thinking the possession thing. 
 

Both channels are experiencing the volume change? While I have never heard of such a thing... if it is in one channel, I would suspect a tube. ARC is really well made... so the most likely culperate is a tube.
 

ARC will have one tube for both channels or two or more per channel. If the problem is in one channel. Then switch the 2 or four tubes per channel in the Amp first and see if the channel changes... then try in the Preamp. If it is both channels, get a replacement and se if it goes away.