A friend of mine had the same symptoms, I told his it was the exhaust valve, about 4 mechanics said it wasn't, when he went to vauxhall for an MOT they told him his emissions were wrong and the problem was with the exhaust valve. The problem in the first place was due to the fuses/resistors which you hook up to your IAT and tell the ECU that the temperature is right, when in fact it is not, so more fuel is injected to compensate and hence, a massive headache.
This might not be your problem, however check if you have a very greasy exhaust, remove the respirator on the filter hose and check if you have it clogged up with oil also.
Personally I would pour a bottle of that cleaning stuff into my tank (follow instructions) to clear it out see if that helps.
I run 2 petrol and 2 diesel cars and do this once every 2 months, they all run like a charm.
V8 Drift Champ, September 2006
