I'm hoping this is the best section for this, as I've seen this problem posted many times.

My A4 1.8 was wildly fluctuating between 600 and 1200 revs at idle.

I was gonna remove and clean my MAF (mass air flow sensor) after reading all the posts, but once I'd looked down the pipe with a torch (sorry I'm no technician!) and everything looked cleanish, I decided to leave alone and instead looked at the other end which goes into the carb.

It's here I found a butterfly valve which opens to let air into the carb (probably!?).
It was marbled with oil, so I simply sprayed loads of carb cleaner at it and re-assembled the hose/duct (whatever!).

Result: Idles beautifully at exactly 900 revs and a definate 10-15% improved performance thro' all the gears.

Hope this helps as a first course of action instead of going straight for the expensive bits!

 

something worth remembering indeed, such a simple fix

 

You cleaned the throttle body.