First suggestion, always: exercise the pots and switches. Turn the puts from fully closed to fully open, and back, like 50 times. Move the switches through all positions (some have 2, some have 3, a pickup switch has 4) again like 50 times. The pots are fully enclosed (so the usual spray-on stuff will not work) but they are self-cleaning - as in: if you don't use them then this problem might build up over time.
If that doesn't help then some of the solder joints may have gone bad - which shouldn't be a terribly expensive thing to fix. Any electronics package that isn't a full-blown Series I or II should be easy to remove from the instrument (pickups and subassemblies use click-on connectors) and send to the Mothership for some TLC - make sure to contact them up front for expected turnaround time and costs.
If you're considering a Series instrument from before roughly 1995, and it has these symptoms, then it may need a bigger kind of upgrade that requires sending the whole iinstrument back to the Mothership. The reason is that the older electronics weren't designed with today's RI levels and frequencies in mind, however some of the older instruments seem to be humming long nicely without the upgrade.
