[djbh]

Hey congrats BC.Told you so.Knew you could find the problem by simple elimination.
As for the ballast,check the wiring,look for scrapes and dammage to the insulation of the wires.
As I pointed out before,GFI are very sensitive and will trip on humidity.Maybe the sockets get humid,the humidity creates a pathway for a GFI fault.
If it is the ballast and probably is,garbage can is a good place for it.
Hagen sells a small electronic ballast(GLOMAT) for running 2 30 watt bulbs,comes with watersafe caps and it is cool,does not get hot,as most regular ballast will.Cheap
you can get one for 50$ pre-wired,just plug it in.
Dan