Proton Amp occasionally doesn't play left channel
I have an old Proton D540 amplifier that has recently started acting up. It occasionally cuts off the left channel, or gets really staticy sounding. It doesn't happen all the time, but it's worst when I adjust the volume. I can temporarily fix it by pressing the stereo/mono button several time on the front of the unit. Any ideas as to what the cause could be? I've checked all connections and everything is solid.
Since I'll probably end up replacing the amp (unless it's inexpensive to fix), what would be the equivalent of the Proton D540? This is for a dedicated music-only room, so I don't need video or and A/V receiver.
Thanks everyone!