I used to have a JCM 900 and had both of those things happen to mine. I can't remember now what the problem was that caused the volume thing, it was like 8 or 9 years ago, but I do remember having it re-tubed at that time. Can't remember if that's what solved the problem though...

And for the footswitch, it got a short in the wire, so I had to get the wire replaced that runs from the pedal to the amp.