ESP warning light from cold

I've recently purchased a 2001 A4 Avant 1.9TDI. All is well except the ESP warning light consistently comes on after about 8 minutes of driving from a cold start. The power is then reduced causing shuddering at lower revs. After about 20 minutes, if the engine is turned off and restarted, (once the engine is warm) the problem generally goes away and only returns when starting from cold.

I thought it could be a wheel sensor but can't figure why it seems to work ok when warm. I'm checking it in to the dealer next week but want to make sure he doesn't try to charge me a horrific amount for what may be an easily rectified problem.

Forgive me if this has been answered elsewhere in the forums - I did check but couldn't see a similar issue.

All help appreciated.

http://images.google.co.uk/imgres?imgurl=http://www.kfztech.de/images/kfztechnik/esp.gif&imgrefurl=http://www.kfztech.de/kfztechnik/sicherheit/ESP.htm&h=417&w=617&sz=45&tbnid=E_8khkH2ZlEJ:&tbnh=90&tbnw=134&hl=en&start=1&prev=/images%3Fq%3DESP%26svnum%3D10%26hl%3Den%26lr%3D

it could easily be a sensor as the abs is linked and it could be a wheel bearing or excess play in a wheel "check the lug nuts are tight"

I just bought a 02 A4 and I"m having the exact problem in the mornings. Did you ever get the issue resolved ??