There are a lot of things that affect the Zetec idle stability and even when perfect its not that smooth

Idle Control valve (if your car drops to 1000 rpm on lift of and then to 850 when staionary this is working

Throttle Position Sensor

Exhaust Gas Pressure Differential Sensor (tuns on and off EGR valve, fails and leaves EGR on at wrong times)

EGR valve. Sticks open, clean it

Rubber T piece below coil splits easily and is a vacuum leak (Very Common)

Pulse Air Injection system. Sticks open after warm up

HT leads break down very easily

Too large a spark gap. Try 1mm its a lot better

Crankcase Breather valve, Stalls the engine when blocked

So you have a lot to check

Mad Mole
Visit Mad Moles Mondeo Madness
2L Zetec, Cams, Stage 2 head, Valve work, Powerflow, Unichip, AEM Ram, K&N, Ecotek, 75mm Lower etc.
192 BHP, Faster than a speeding Duratec!