Could be poor charging of the battery from the bike.
Does the starter work properly with a freshly charged battery?
Testing the battery charging.
Get a multimeter and check the voltage of the battery. A good battery should read something like 12.6 volts or higher. If it gives a good reading the battery is fine. Lower it´s suspect or needs charged.
If the battery is low, take it of the bike, charge it, leave it standing for a while and check it again.
Once your sure the battery is good. Put it back in the bike. Start the bike. Connect the multimeter and run the revs up to about 3,500.
The meter should show about 14 volts going through the battery. Try it again with the lights on. If there isn´t a charge of about 14 volts going to the battery the regulator may be suspect. Check it and clean the connections. Test again.
http://www.vespamaintenance.com/elec/charger/index.html