The idle speed on my 98 sti4 is playing up. was ok, but now idles mostly at 1200 rpm, occasionally returns to normal but not for long. Any ideas?

 

From experience (other peoples experience not my own) I would check:

1. Lambda sensor
2. Idle Valve
3. TPS
4. Crank Position Sensor
5. Coolant Temp Sensor
6. MAF (not likely on a '98)

If you can get it connected to a Select Monitor you will be able to tell if any of the above sensors are duff or not.

Andrew...