I had severe jerking and hesitation with my car and it turned out to be a dodgy injector, but that would affect running at higher revs too.

Does sound consistent with sticking EGR valve from what other people have said on the Ford forums. Cheap enough to get it checked out and maybe cleaned. As the previous poster pointed out, if the valve moves freely perhaps the actuator is not working properly?

Also with a fly-by-wire car like the Focus there's a potentiometer in the accelerator pedal and if this is a bit flaky it could be affecting power application, though I've no idea if this is a common fault on the Focus.

I guess you can get the fault codes read when you take the car in to see if there are any logged faults. Might not catch an EGR issue though.