[Ruffy]

At the risk of "jumping on the bandwagon", I too would suspect the regulator if, as you say, the wiring is OK. I suspect over voltage (therefore driving too much current, which is a nightmare for filament bulbs) which is what the engine alternator will produce when run at speed, easily up to 15V+ unregulated.

Do you get the same problem if you leave the lights on without the motor running? If not, this would add weight to the over-voltage theory. If you've got time to recharge (as, unfortunately it will run the battery flat quite quickly), perhaps try it one evening or overnight to see how long they last.