My 2wd sapphire has developed the usual erratic idle problem, the revs now go up and down when the engines hot or cold.

When hot if I unplug the idle control valve, the engine speed stabilizes. I've gaped the pluges and checked the fuel pressure, reads 51psi at idle. I've also checked the TPS with a volt meter, got a reading of 5v. Can I assume that the idle control valve is at fault? I tried cleaning it, hasn't helped. I don't want to spend £100 on a new ICV just in case it's something else.

All advice is greatly appreciated. I'm new to Cosworths and a bit out of my depth.

P.S. The CO was set recently at MOT station, the plugs however are a bit sooty(Champion QC57C).