1.9s1

Hello all, my car has developed a problem and i have no idea what is causing it (typical really as i was planning to sell it soon and replace it with a focus st). It starts first time with no problems, idles perfectly but when driving at low revs its ruff as hell missfiring. if i keep the revs up and slip the clutch it will run ok but this is not something i want to be doing too much off. once it has warmed up it runs spot on. I've tried the usuals plugs, leads, even iscv but not getting anywhere. this is a driving me mad as it has been running lovely for years on the EFI setup but has now developed this fault. Any help or advice would be greatly appriciated.
Thanks
Mark