In my opinion it could be a number of things!

1) Air leak around inlet manifold/anywhere after throttle body
2) Idle Speed Control valve (as discussed above)
3) CO setting on the ECU has been messed around with (mine had a very similar problem and this was the cause, get it to a specialist as if it has been turned down to pass an MOT the car will run lean throughout the rev range and could melt!)
4) Another sensor has gone funky, could be coolant temperature sensor or throttle position sensor.

Most likely to be 1 2 or 3 in my opinion!