i have the mk3.5 rs1800 and when the cars gets even slightly hot (couple mile drive) the idle speed will drop to around 200rpm then back up to 750rpm and do this several times, it almost stalls its self. Also when warm, the car will hit a flatspot between 4000 and 4500 rpm which is pretty annoying to say the least. i have replaced the idle speed control valve and mass air flow and im wondering what else it could be? i know the exhaust system is a bit knackered so im replacing that and i have been told it could be the lambda sensor? any ideas or advice would be much apprieciated, cheers, dan.