Did you ever get this sorted? My 2002 1.6 (Zetec SE) has this problem sometimes. I've tried a new idle control valve and PCV. No splits apparent in vac hoses. The Ford garage diagnostic showed no fault and after more than an hours labour cost they found no reason why it should happen.
I had the cam belts replaced not long after I bought the car and the fault has been only since then - but I cannot imagine their is a connection.
Replacing the speed sensor?