I'm assuming that both speakers are going up and down together, therefore, I'll rule out a problem with the speakers themselves. Here is what I'd try:
1.) Hook up speakers directly to the receiver. If the problem doesn't happen with this set up then you can rule out the receiver as the source of the problem.
2.) Check all connections to make sure they are all tight ... especially the ones in the wall volume switch.
3.) Replace the wall switch. My reasoning is that after the two steps above, this is the only thing that could cause the sound level to go up AND down.

Good luck.