Once my car is warmed it, it idles at only 590rpm and feels pretty rough when I'm sitting in traffic or even manoevring to get into a parking space. The car vibrates.

When the engine is cool it idles perfectly at between 750-800rpm. Car has about 42K miles on the clock. In all other respects it runs fine.

I've been back to my Audi garage several times but have so far been unable to improve things. 

I'm concerned that I am somehow damaging the car by using it like this. Reverse often feels pretty bad.

Any ideas what it could be ? Would changing the calculator do it or is it more likely a dirty sensor somewhere malfunctioning, or maybe something else ?

Hi David....welcome to the Forum

I would imagine a low idle speed is nothing more than the induction system being clogged up with carbon.

Probably best to run some cleaning additives through the engine to clean it out.

More efficient than that is TerraClean treatment http://www.terraclean.co.uk/ which should really sort it out.

Let us know how you get on with it

Cheers Trevor