I have a similar problem:

When I start the engine it runs at about 2000rpm (normally about
1000). After a minute or so, or once I start driving, the revs
oscillate between about 1200 and 1800 every second. The problem is
only when in neutral - with any load on the engine it seems Ok. To
hold the car stationary, I need to keep it in second gear with the
clutch half out and the brake fully depressed. That keeps me
stopped and the revs stable. I can then pull away (in second) with
no accelerator.

Seeing your replies suggests mine may be the same thing... but which cables do I need to check and lubricate. Bear in mind I'm a total novice and engines are a bit of a black magic to me (I'm an accountant...!).

Thanks in advance,
Richard