[forchetto]

Slipped ignition timing could cause this, but the only way would be for the key that fixes the position of the rotor to shear in the crankshaft. Unlikely.

Also unlikely is the valve timing slipping by the cam chain jumping a tooth.

A lean mixture can cause this. Try richening the low end by tweaking the mixture screw. If that doesn't do it I would check the valve clearances. A tight inlet valve could cause it.