Fiesta MK 4 engine stumbling?

My Fiesta MK4 1.3 Endura engine tends to stumble/hesitate when accelerating at low revs. I recently had the coil, ignition leads, spark plugs, fuel filter, throttle position sensor replaced, but it is still there. Any ideas. It tends to be fine when the revs are higher. I've also cleaned the fuel system/injectors with a petrol additive.