When I cold start the car is starts without problem and idles nicely 8-900 RPM.

Then once it's been running for a 4 or 5 mins when the engine temp get nearly halfway, where it usually sits, the car idles lower, maybe 6-700 RPM.

And once it's up to normal temp it it idles around 500 RPM. It seem quite low and feels like it's going to stall. It hasn't yet, but that's mainly because I don't leave it idling too long.
Any thoughts on why this might be, or what might be done to stop the engine revs dropping once the engine heats up?
Many thanks,
Paul