Could be the switch or some wiring going bad. Sometimes you can clean the switch up temporarily just switching it back and forth rapidly to clean the contacts. If you have some contact cleaner, spray some on. But it's temporary. This happened on my SG (within a few months of purchase) so I replaced the switch and the pots while I was at it. No problem since then.

If it's a wiring problem, could be bad ground or just a bad solder point. Try finding a bad joint (probably will look dull) or just resolder the points along the path to the neck pup.