Totally remove the throttle cable so the throttle can be freely rotated by
hand.

Gently move end stop to end stop (especially at the nearly closed end)
feeling for any drag, snatching etc.

The other possible cause is that the ECU which is self adaptive has got
itself  all confused and needs retraining.

The sequence (from memory) is as follows:

1) Disconnect battery for 30 minutes
2) Reconnect bat.
3) Start car and allow to idle till fan cuts in 3 times
4) Rev car at a steady 3000 rpm for five minutes
5) Release throttle and allow engine to idle for 1 or 2 minutes
6) Switch off

Please report back how you  get on.

Nick /////

My girlfirends Punto started having an erratic idle. I have changed the spark
plugs and air filter. Since then, the engine now holds revs after about 7
miles of driving. It either idles hunting between 800-1200 revs or holds at
1500. If it does come down to 800 it then stalls. The revs are held at 3000
when you come off throttle. I'v checked the throttle cable, lubricated it and
its not sticking. I've also cleaned the stepper motor out with WD40 hoping
that would cure it. The engine when under load is fine but when you are
feathering the throttle it runs lumpily and the car is jumpy. I took the car
to Fiat, they checked the ECU for fault codes and there was nothing stored.
They flashed ECU with an update of software and it didn't do anything to help
out. Please would someone point me in the right direction as to how this is
caused. Thank you