Intermittent high idle

Sometimes my car decides it wants to idle around 1500 rpm. Today it started fine, idled around 1000 just after starting, then as soon as I moved it forward and stopped to close the garage door it went up to 1500 and stuck there. Once I'd driven for ten minutes and warmed the engine up it was fine, dropping back to about 800 when idling. Then as I was coming home from about an hour's drive it started idling high again, so it seems to be fairly random and not dependent on temperature. I managed to pull over and have a look under the bonnet while it was still idling high, but I couldn't see or hear anything unusual. Tapping the accelerator doesn't do anything.

Any ideas? I'm thinking of changing the vacuum hoses. Anything else worth checking?

1985 Cabrio, 1.8 DX engine, running perfectly apart from this problem

The idle bypass valve could be worth a check, maybe the connection to it is poor causing your probs. It is located on the back of the inlet manifold on the same side as cold start injector. It is injection isn't it?

Yes, it's injection. I'll give the bypass valve a check over, thanks.