My project car does that maybe once in every 20 start ups from cold. Just after the starter motor disengages it does a 'boy racer' burst of revs with the three peaks like you've just described for a second or so, then back to normal, and runs completely normally thereafter. No obvious reason why, no errors logged etc barring a very occasional "coolant temperature sensor open to ground 'intermittent' fault". I wondered if I was being a bit hasty starting rather than giving it the second or two for the glowplugs to preheat on occasion without thinking but no other ideas. For the revs to happen there has to be fuel but where is it coming from?