it is hunting wildly (nearly stalling) perhaps spluttering at low revs when driving then its most likely the Idle speed control valve at fault. These can be cleaned up, failing that a new one from fords is about £90. Quite a common fault on these cars. Bad coil pack wiring, coil packs can also cause this. If its just a mild hunt at idle i wouldn't worry about it to much. Mine occasionally does this from time to time for no particular reason.