Having a problem with my P1s idle when cold it goes up and down from 800 to 1200rpm when it has warmed up its not so bad but still does it occasionally if you rev it up to say 4000rpm and take your foot off the throttle quickly engine almost cuts out it drops to about 500 rpm then it picks up ok. HELP

 

MAF perhaps?

 

Have you cleaned your maf?
Mine had the same symptoms until I cleaned it

 

Do you have a blow off valve? It can cause similar symptoms especially if it's leaking.

 

had simula problem with my 04sti it was the idle control sensor. toook it to subaru then they got the code from the ecu dinosties and they jus reset the ecu and its never been a problem since but onli done 800 miles with the reset ecu. cost 40 quid not bad as it was spewiying out loadz of expensive v-power