I own a 133k mile Fiesta which has had an intermittent misfire / hesitation for as long as I can remember. Basically this occurs between 1800-3000rpm, usually in the higher gears 4th / 5th etc when running up a slight gradient or into a strong headwind. It's more likely to happen in damp weather. The mechanic I use says that he cannot find any error codes and cannot replicate the fault and until the engine management light comes on to just keep driving the car.

I have been told that some error codes remain in the memory and some don't. One final point - when the misfire / hesitation occurs the tacho twitches noticeably as if it is losing its pick up. I wonder if this and the fault are directly linked. It is a great little car, very reliable but I'll admit has not been serviced by Ford and therefore may have missed out on any software upgrades.

Cheers