Erratic idling when hot

When ever I am stuck in traffic for more than a couple off minutes the tick-over drops then races, then drops then races etc etc. Even slightly putting the throtle down doesn't really help. Any idea? The car ticks over OK at other times. Car is Golf MK1 GTI Cabby.

Symptom: Irregular idle with warm engine

Cause Remedy
Leaks in air intake system (false air) Check air system for leaks
Sensor plate and/or plunger not moving freely Check for free movement
Cold start valve leaking Check cold start valve
"Warm" control pressure too high Test pressure
"Warm" control pressure too low Test pressure
Injector leaking, opening pressure low Check injectors on tester
Injected fuel quantities unequal Check for equal fuel quantities
Basic idle setting incorrect Check and adjust CO level



Copied from below , no definite answer but my favorite is air leak somewhere

Thanks for information - a busy weekend ahead.