03 A6 fast idle, stall, ASR indicator
 

My 2003 A6 with 117K miles periodically (now more recently) has the ASR indicator light come on simultaneously with the RPMs revving high then so low that the car almost stalls (and the battery light comes on briefly). After about 10 minutes of continuous driving it seems to go away. Especially a problem when I come to a complete stop...car almost stalls-out. My mechanic says it could be a lot of things and no codes come up. I'm not a knowledgeable car guy - just looking for some advice that I can relay to a capable mechanic. Any ideas?
Thanks
Peter