ticking over to high

Hi guys wonder if anyone can help me, my 89 cab gti 1.8 injection when i start it from cold it ticks over fine and as soon as i start driving and it starts to warm up it ticks over 2,000 to 2,500 rpm any ideas
Thanks Chubbs

Aux air valve stuck open?

Thanks mate i will take a look at the weekend
Chubbs

It could also be a leak on the vacuum pipes somewhere.

Thanks for advice ill let u know how i get on
Thanks chubbs