Yamaha WR450 charging system diagnosis 

A bad rectifier would cause voltages to go up and down and not the steady 12-14 volts. Anything above 14.5 volts could cause bulbs to blow. On the stator, look for discoloration of the windings. Ohm reading of zero shows a short.

Bike still has that stator. Works good

I think you should check your ignition coil first.