my 1997 312 sprinter keeps "holding back" when accelerating,doing it all the time now and has got progressivly worse,when you brake and the speed drops below 20mph the abs starts working,anybody know whats wrong with it?ive changed a rear abs sensor that was damaged,it was fine after that for about 100miles but has got worse again,any help greatly apreciated as we have to get to rockingham in the peice of shit on saturday for a rally.