Engine warning light 01 GTI 150bhp

[fixedgear]

Hi,
I was wondering if any one else has had similar problems.
I have a 01 Golf GTI 1.8 150bhp.
The engine warning light keeps comming on and then after a while it will go off (perhaps a few days or a few weeks)

I've had the Lambda sensor changed and the throttle boby cleaned.

History
Car became a little sluggish, some time pulling off from junctions it would just chug accross the road. The only solution was to ride the clutch and keep the revs up a little.
Some times when driving it would feel lumpy or become a bit choppy. Usually this happens after dropping down a gear ot two and overtaking (35 -70mph). The car would then run a bit rought throught out the full range even when cruising.
I have had the car at an independent garage a few times and they have read the fault codes some of them are not defined. Brought it to a VW garage and the same story, they recommended cleaning the throttle body ( a problem on similar 1.8 passats)
The car now feels much better and is running smooth, but the light came back on last week and is still on.
The independent garage says that there is an air valve down at the turbo that may need to be replaced, they have disconnected it and no fault showns up on diagnostics. They think it might be the cause.

Has any one had similar problems with this engine.

Sorry for the long post, it is also my first one here.

[Eshrules]

first thing id have done is get it on a diagnostic system, your working blind otherwise, you could go through every single part and never resolve the problem, you need to know what kind of engine codes are being thrown up to narrow down your area of work. you can get a code reader for around £40/50 yourself nowadays, id invest in one, their invaluable