Could someone help please, for the last couple of days our 1998 Escort Gti has been trying to stall on tickover, when you first set out in the car it seems to be ok but after 5-10 mins in the car when you put your foot on the clutch the revs seem to drop right down and then appear to bounce back up, this will continue for a while and the revs seem to bounce up and down trying to stall then it seems to level out again, could someone please give me some idea what is wrong?

Many Thanks in advance

Jobbo