I know this has been covered before, but I can't sort this one out.
My idle has become really erratic (revs up and down) to the point where is sometimes stalls at the lights. I've run 2 cans of upper engine clean, removed and manually cleaned the idle valve, and also done an ECU reset but it's still the same. I have run the tank right down and refilled it with a full tank of ultimate and put in a bottle of octane booster but it's no better. Occasionally it will idle fine, but most times it's like "bruuuuum, bruuuuum, bruuuuum"

I don't know what else to try without spending loads of money trying parts.

Any ideas????
Do idle valves play up on the GC8????

The car drives fine it's just the idle.......

SC TER