Amp issue, terminal or simple?

Hello everybody.

I have a quick question regarding the health of my amplifier. Firstly its a Denon PMA-350/ii from around 1993, it has outputs for two pairs of speakers 'A and B'. The problem is that the left channel on 'Speakers B' produces little or no sound when playing music. This can be correct, however, by switching 'Speakers B' off and on again several times in a row, or by increasing the volume slightly, then the speaker will suddenly boost in at normal level. I've tried different speakers on this channel and different music sources too, but the problem still persists in the 'B' left channel. There's no issues with any of the other channels or any other area of the amp (that I'm aware of).

I was wondering if anyone had any knowledge about this problem and whether its terminal, or something quite simple and common. I've had the amp for about 4 months now (Bought used of course but in overall great condition for its age), and I've only just noticed the issue in the last few weeks or so. It may have been there prior to purchasing it but I wasn't aware of it. (I only payed £60 and got a 12 month guarantee with it anyway if something can't be fixed).

Thanks to any replies.
My first guess would be a bit of corrosion on the volume control. If so, an easy fix would be a touch of "tuner spray." Or otherwise, you could try replacing the volume control, which could get a little expensive.

My second guess would be a spotty capacitor somewhere, given the age of the unit. Capacitors become suspect in 20 year old electronics. The bad cap could be hard to identify, so it could be a trip to the local service shop. By the way, after they locate the bad cap, I would urge you to consider having that cap replaced in both channels, since the other channel could be close to retirement also.
Radio Shack carries a cleaner/lubricant spray. As James said, remove the cover and spray this into the switch, with the power cord disconnected from the wall. While it is wet, operate the switch serveral times and allow it to dry, reconnect it, and see if it cures the problem.

If not, and you want to hook up two sets of speakers, but don't want to individually control them, you can hook them both up to Speakers A. Functionally, it is the same thing.
SOme sort of problem with the switch. A cheap switch is next to nothing
Otherwise i am certain your amp is fine (aside from the switch)
+1 on the contact cleaner, but also try it on the A and B speaker switches.
My guess is the output relay has pitting on the contacts. When you advance the volume control, more current flows through it and it "kicks in"
Your best bet is to remove it, remove the plastic cover and then clean the contacts with a burnishing tool. After you're done, treat them with DeOxit contact cleaner.
Good luck.