My '06 Astra Sportshatch Sri seems to have an intermittent fault in the ABS and traction control system. Occasionally, when starting, the warning lights stay on for several minutes and then go off. Sometimes when driving along they come on, stay on for a few minutes and then go off again. As the car is still under warranty, it's been back to the dealer twice who, unsurprisingly, can't find anything wrong. It was in for a service today, so I asked them to check again. I was told the ECU checks out fine. Anybody got any ideas what might be causing this?