over-revving from a cold start

anyone have the problem of engine over revving when cold starting your mpv? mine has always "roared" to life but since a head gasket job my mechanic can't seem to get the computer tuned properly...it revs to 2500rpm drops after 30sec and from time to time when starting depending on the engine temperature I suppose? goes to 2500 then fluctuates 3 or 4 times rapidly almost to stalling. Cold acceleration "pulsates" as if the engine is getting too much then not enough gas. <br>Could this problem be related to the oxygen sensor?<br>any help or suggestions would be appreciated

I have faced this type of problem several time. But I don't know the exact reason behind it and it is still happening the same thing.