This is a long shot but if everything else fails get a can of contact cleaner and spray the volume and tone pots. If you are a lot in dusty enviroments the wipers in the slides and pots tend to get coated with residue and do not maintain the proper contact. I have run across this on rare occasions. Having said that however, my first guess, like all the rest, would be voltge surges. They are quite common and will cause the problem you describe.