V6CDTi - Erratic idling

[Celes]

Recently the car has decided to rise its idle to 1000rpm. Will drop down to 700rpm if I put it in gear and drive around in 1st, but will go up again when in neutral.
Sometimes when the engine is cold and I change up to second, but keep the clutch down, the engine rpm drop to 550rpm for a few seconds...

Anyone got a clue?

[roswell]

crank shaft sensor may be on the way out, but before that check all your vacume hoses

[Celes]

It's done it twice today when the engine was at 70°C.

All hoses are OK and the engine still pulls like a tank.