Got stuck in traffic on the way home from work which I think is what started this off.
Engine idling in traffic, temp was showing at ~90deg, suddenly the revs drop off to just under 500rpm, the engine feels like its running rough/lumpy and the engine warning light comes on. After about 3 seconds the revs rise back to the normal 800rpm(ish) idle and alls well. This happened 5 or 6 times til I got clear of the traffic and the engine cooled to its normal temp (around 80deg) then it seemed fine again.
Any ideas? The only mods the car has are a Courtenaypanel filter and a Vectra intake, can't see it being anything to do with that?