My Mondeo used to idle at just under 1,000rpm on the rev counter. Now, when I start it up, it goes up to 2,500 for a few seconds, then comes down to just under 2,000rpm.

When driving, when I come to a junction, the rev counter goes up past three and at other times when stationery, the revs alternate back and forth from 2k to 2,500.

Also, another problem, difficult to describe but I think it may be related, when using cruise control, it feels like it's running out of petrol. This is the cycle:

1. The cruise control accelerates,
2. car goes,
3. then feels like engine is starved of fuel,
4. The cruise control accelerates, etc.

My question is, What would cause the engine idle speed to go so high, and how can I stop it?