Fiesta 2000reg 1300cc Engine Problem

Guys Fiesta year 2000 1300cc saloon petrol. Engine revs rise
then take a while to drop, when driving you lift your foot off but engine continues
to rev high for a while, I suspected the stepper motor, did the usual a good
seeing to with a cleaner and then a good dose of WD40, replaced and fired up
took it for a spin and yes much better, daughters car so told her if it sticks
clutch in give the accelerator a good up and down and should help, got a call
dad car is no better so bought new stepper motor, fitted it but engine worse
than ever with new stepper, so while new one was on the car I gave the old one
a really good seeing too, refitted and car was allot more settled, drove it myself
a distance of 60 miles and for me it seemed ok, until nearly home then it began
to play up again, this time I went in search of and air leak, and found a lug
on the inlet with nothing connected to it so IU blanked it off and started
engine seemed ok, so daughters asked if I would fit new stepper again, I did
took it for run and again seemed ok, daughter took car home phoned and said car
the same as was. Scratch my head nearly down to the bone, anyone throw any
light on my problem please.

Obviously to technical question!! answer is the carburettor
position sensor out of sink.