anybody up for a guess at whats the problem?!

cold starts lumpy sometime cuts out after a few sec, then when it does run its at 750 rpm and stutters abit, when driving its ok up to between 3.6-4k then big missfire/ rev limiter type feeling! if I blip the throttle at tick over it sometimes cuts out.

Its at NMS this week being sorted so will post up answer asap, whats your guess?