2002 Octavia cannot hold at 2000rpm for NCT

Hi,

My 2002 Octavia (1.6 petrol) failed the emmissions part of NCT as they couldn't get it to stay at 2000rpm when idle. When the engine is cold it will idle at 2000rpm, but once it heats up, then once you try to rev it above 1500rpm it hunts up to around 4000rpm and if you ease off on the accelerator at all, it falls right back to about 800rpm.I tried cleaning the coolant temperature sensor and tbe throttle control body, but this made no difference.When there is a load on the engine it runs fine and the throttle control is fine.Any suggestions as to possible causes would be much appreciated.

Thanks,