Does this happen all of the time, or just when you start the engine from cold?

I had a problem with a Ford Escort 1998 model 1.6 zetec engine. If you started the engine from cold, if you pressed down the clutch, the idle speed was around 2000 rpm, the car would literally drive itself if you took your foot off the accelerator!

The problem turned out to be the 'throttle potentiometer', which was giving a false signal to the 'ecu', making it think that the accelerator was in a certain position when it was not.

This device is mounted on the side of the throttle housing, and is connected to the accelerator cable mechanism, this might be your fault, but cannot be certain.

It could also be the 'vehicle speed sensor' that is faulty.

Have you got any more information on the problem?