[Arran]

It could be silver migration, depending on whether this uses slug tuned IF cans, but it may be just be intermittent contact. I had to renovate a mini IF can from a 50's radio where that was the problem, not much static but I did experience a volume drop when one of the IF cans got bumped. Before performing major surgery try giving the band switch a good cleaning as well as the tube socket contacts. If it does turn out to be bad micas in the IFs make sure you find out the correct values first, they are not all the same. You also do not need to use silver mica caps to replace the originals, precision ceramics will also work, are easier to find in any size, and are much cheaper.
Regards
Arran