same exact thing happens on mine.. I suspected a bad silicon rectifier but after trying two others and getting the same results, I'm sure it's just normal magneto behavior. There is no voltage regulator.
i replaced the 6V bulbs with 12Volt.
The 6V-5watt and 6V-10watt can be replaced with 12V 10W and 12V 20W (or close to those wattage values). Light output is about the same and they dont fry at high RPM.
A large autoparts store had the 12V bulbs with the correct base.. and they were pretty cheap, like a package of 2 for $3.