pardon me for going off earlier, but 150 psi on a stock vespa is unheard of. Id spray some premix in the plug hole and drill start, thats after cleaning and setting points again and again.
Spark on a vespa goes thru your brake switches and your brake bulb to ground so wise to check those components as well.
Sometimes when a motor first get put together its a bit off...bearings, case halves, new seal, etc. I usually take a smalll block of wood and whack it a few times and then spin to make sure the only resistance is the new seal, if its periodic like you say then check for rubbing against the subframe or worst case scenario is you dropped the crank or nicked the crank lobe and everytime it passes your intake patch its gouging it up, terrible.