I sent an answer but it seems to have disappeared! I presume it`s the Sigma/Zetec SE engine. Has it just started or has it always done that? What speed does it idle at when hot? Sounds like idle mixture is wrong......could be several things......Lambda sensor failing (in exhaust)......air leak in inlet manifold ( rubber tube split, blanking cap leaking).... Error in throttle potentiometer setting (it is very sensitive at idle setting position)......Does it pop and bang a lot when lifting off throttle? Does it have high fuel consumption? Does it `kangaroo`/ when driving off from low speed/stopped?