Treble control drifting on speakers?

I've got this (relatively old) set of speakers from an offline desktop I used to have. They've been sitting around doing nothing for quite some time, so after uncovering them while rearranging some stuff I decided I'd go pick up a digital audio (coax) cable and give them a shot. They sound considerably better than the speakers I was using before (the addition of a sub is very nice). The problem I'm having however is this: while trying them out I suddenly started noticing the volume levels out of the two main speakers, particularly in the upper range, seems to drift randomly. I should also note that there's a treble control on the speaker next to the volume control, which doesn't seem to be affecting anything. Any thoughts on what's causing this (and, hopefully, how to fix it)?

I also use an old set of speakers. I have found that if the wire is bent a certain way, the connected speaker does not play, and when this is a tweeter, it results in less treble. Maybe your wires are bent?