Hi.
I have the same problem with my 1998 A4, although the light now stays on almost all the time. I have just had this checked by Audi who tell me it is the ABS Unit. The computer was checked for error codes but none were found. Apparently the braking system will run without any problems - its just the ABS unit which is faulty. I have booked my car in to have a brake fluid change in case there is any debris in the fluid. I have nothing to lose!
I think there are places which will repair the abs unit but have not found one yet.