Low Beam keeps blowing.

I ride a 99 model Honda VFR800 motorcycle.

The other week I noticed that both the low beam headlights on my bike weren't working, high beam worked fine. These were the original stock Honda bulbs. I could see bits of filament floating around inside the glass.

I replaced both bulbs with good quality Philips H4 globes, tested them and both low and high beam worked. I rode only a few km's and noticed that both low beams had gone again. Hi beam still worked. I put an old globe in and low beam worked so I assumed it was low beam blown again in the new globes.

I took the globes back and the accessory shop replaced them.

I pulled the front off the bike again, tested the relay, ( I think I did it right ) and it was OK, both the high & low beam relays are the same so I swapped them around thinking that if it was a faulty relay, high beam would blow this time. I checked for worn wires that could be shorting out, but couldn't find any, which is not to say that there isn't a short somewhere. I tested the voltage output to the headlights and it got to the high 13's when revving the engine, but never reached 14 volts. I installed the second set of H4's ( same as the other new ones ) turned them on and both high & low beam worked fine. After only going a few km's both low beams had blown again.

I assume it's not the relay because I swapped the low and high beam relays around.

I think that the cause could be one of 3 things, a spike in voltage, a short in the wiring somewhere or perhaps the dimmer switch ?

Any thoughts or suggestions would be greatly appreciated ?

Thanks

David