Voltage regulators are not a known failure point on GTR's. I would look at it, but would also look at mechanical factors, like a loose front fairing stay bolt or damaged or incorrectly assembled headlight bucket insulator rubbers.
Also, are you replacing teh bulbs with the same type/brand each time, and if so, which brand?

Regards, Andrew.

 Vstrom is gone, on the hunt for a GTR.