My wifes 240 wagon ('93) abs system will activate once or twice after every start, when reversing out of the driveway. I can start the car, back down the drive way, then apply the brakes, and the pedal thumps once. If i release the brakes, then reapply, everything is normal. If i stop the engine, immediately restart, and continue backing down the driveway, when i apply the brakes it thumps again. It has been doing this off and on for a couple months, and I cleaned the grounds on the abs unit. This temporarily made the problem go away, however now it is back and very consistent. It also randomly acts up when going forwards at slow speeds, the abs system will kick in and the pedal will vibrate as expected if I was on ice etc. The pads/rotors are 6 months old, pins lubricated and in good shape, no abnormal vibration during braking, and the other day when someone cut me off, I slammed on the brakes and the car stopped as it should. Any thoughts on where i should look next?