Rough Running At Low Rpm

Hi,

I have a persistent (i.e. our local garage has failed twice to fix it) problem with a 2008 Fiesta 1.4 petrol. The engine seems to have a real rough spot just above idling revs. At idle it is fine, and it always starts no problem. At 1600RPM+ it is fine; it never misses a beat. However, if in slowing traffic on a level road, I back off the throttle, it often becomes rough around 1200-1500 RPM, even if there is very little or no load on the engine.

It does this hot, cold or in between, but is probably worse when the engine is part-warm.

I believe modern engines such as this one cut the petrol completely when you take your foot off the accelerator, and then some mechanism kicks in when the revs drop to idling, to prevent the engine simply stalling. I could be way off the mark here, but does the oxygen sensor have anything to do with this?

The garage has fitted new plugs, leads and a coil pack, all at great expense, none of which have had any effect at all on this problem.

Any advice would be very much appreciated.

Kind wishes ~ Patrick