Punto 75SX Eratic Tickover

I've had a number of electrical problems with this car but it's running 
quite well at the moment.

EXCEPT: Every now and then it starts revving up on its own accord. The car

starts and run ok but after driving for a while it takes it upon itself to

vary between 1000 and 2500 revs. This looks and sound bad if you're
stopped 
at traffic lights and even more so if you're waiting for someone walking 
across a zebra crossing. There is also an increase in fuel consumption.

Has anyone got any suggestions?

Cheers

Rick