Volkswagen Polo high idle

My polo sometimes has an irratic idle. By this i mean the car will some times hold the revs at 1200 rpm when i'm stopped at a junction and sometimes when i start the car. It doesn't seem to be affecting fuel economy, as since it started I seem to be getting well in excess of 40mpg rather than my usual 32mpg that i get through my normal driving.

My foot obviously isn't on the accelerator when this happens and I've tried pulling the accelerator out to ensure it's not sticky. It holds the reves evenly at 1200 ish before dropping back down to the normal 700rpm.

In the last 2 years I've had the timing belt replaced, the engine recirculation valve, the catalytic converter, three lambda sensor amongst other things. Obviosuly I'm a little concerned that this may be another expensive fix. but so far I've found no similar cases online.

Any suggestions?

Many thanks