My 1994 Audi 100E 2 litre petrol has started to show the abs warning light after about 5 or 10 mins driving. Sometimes it stays off for much longer. If I turn off the ignition and restart the car the light comes on and then go off as normal as the system does its self test. 5 / 10 mins later the abs light comes on. Could this be a sensor problem ? - I would assume that if it was a fault in the abs system then the light would come on permanently. Any help would be greatly appreciated.