Fast idling when hot

Need help with a 1997 1.4L Golf. The engine runs fine when cold but when hot, the idle is 1800 rpm when stopping at a junction. Switching off the engine and turning it back on immediately restores the idle to 800 rpm (so no vac leaks). Waiting several minutes also works.

This version of the Golf is not fitted with an Idle Control Valve nor is there an idle valve on the throttle body. (I removed it from the intake for a full clean)

Any ideas as to how the idle is controlled on these engines?