Bathroom GFCI Troubleshoot
I think it is a safe bet that it is one of the fixtures. You seem to have ruled out everything else. Start with the fixtures that have been burning out lamps.

If you feel confident, turn off the breaker and remove one of the fixtures. It may or may not be obvious if it is the culprit. Cap off the wires coming out of the wall, turn on the breaker and try the switch. Repeat if the problem persists.

Edit: Upon re-reading, when you say "trips the breakers", do you mean the reset button pops out on the GFIs, or the circuit breaker trips?? 
A GFI is NOT a circuit breaker.