hi the imrc should activate around 4500rpm, tic over should be 800 -/+50 up to 1200 cold,

does the st have a throttle sensor /potentiometer as i had this play up on on my escort and only affected certain revs,

the st can be funny with cheep plugs,? check for VAX leaks, and air sensors if you have changed the plugs were the old ones dirty? could be sign of probs,

ive heard on another sight that the ecu can loose its program, or may nead resetting (to reset unplug the battery for 40min, to reprogram is specialist/ford job,
im also told that theres a sensor on the clutch peddle that puts the ecu into shift mode if this is playing up can affect the engine running,
also worth checking earths and connections

(at the moment my cars not well but will live for now, my imrc is stuck open, i get a loss of power round 2000rpm when cold, clutch seems slow to diss-engage (worst for second gear i have to put clutch down n wait a second or 3 before selecting gear or i get crunch, and the air-con clutch died,) not looked into these to much as every time im free it rains, any ideas welcom