Sounds like your switch has a circuit breaker built in. I don't know what year they started that but all headlamp switches after a certain year have a built in circuit breaker. It will get weak and start shorting out. You can't do much to test this but you can look over the wiring system for any obvious damage and replace the switch.
I didn't do it, no one saw me do it, you can't prove anything! "Bart Simpson"
| 
