Does sound like an air leak to me, i had this on a aprilia rs125 i bought on the cheap, the problem was not enough compression in the chamber, once i had replaced the crank seals it worked fine.

Electrical side of things, have u gapped and cleaned the tdc sensor on the ignition unit?

Other then that only other thing i can think it could be is the exhaust chamber, or exhaust can filled up with carbon waste?