Intermittent one channel weaker on input select? Help?

Hi all hope someone might have a suggestion to help me with an Integrated Amp problem.
Sometimes when I turn it on the left (defo not the speakers) channel is weaker than the right, if I then start to flick back and forth between inputs using the remote it comes back strong again?!
Not immediate return some times I have to go back and forth a few times and it sounds like it has 'levels of muddyness' ie I can change input once and it gets worse, then comes back again.
It's a Cambridge Audio Azur 640A (yeah I know) and it's only powering the tweeters.
I have already taken the top off and given it a good blast with rubbing alcohol - to no avail.
It's not a massive problem as a bit of fiddling solves it, however it does plant a nasty seed of doubt when I'm listening to my system as once or twice is 'gone weaker'/popped out of focus (I mean it audibly slipped away as I was listening).
Thanks for any replies!

 

Most of what you say sounds like a dirty/failing input selector but given the fading issue it's not certain. When you say you've taken the top off and blasted it, have you taken the knob of the selector and blasted from the front into the pole gap, or even better from inside looked for a little hole in the selector box and blasted through there? After doing either, or best both, twist the selector back and forth dozens of times, with a pause every so often, and you might solve it.

 

Hi Lawrence thanks for the reply, the input selection is done by numerous buttons on the front panel (not that I touch the front panel - remote control only) so no pots to collect dirt etc
These days it seems to be OK from startup 'most of the time' and when it isn't it's pretty easy to hear and then use the balance knob to isolate, then back and forth between inputs until the tweeters become 'fully bright' (use a reference song all the time). It's just frustrating that if I switch from CD to, say, my Blu-ray I have to check that the one channel isn't a bit duff.... If I had any money I'd just buy a new amp as I've had two of these (I stick to what I know rather than speculation and money spending) both have been problematic.
Ta anyway