I remember having this problem also as a kid, and finally came to realize that my replacement bulbs were not rated for the voltage (6v, IIRC). I think I had 4.5v (3D cell battery bulbs) in place. You can get away with this for a while when two bulbs are in parallel, as their capacity exceeds the max current output of the generator. So as long as you 'load' down the current, the voltage stays in check. Once one bulb fries from the slight overvoltage, the second one gets the full blast, and cooks shortly thereafter.