2000 Ford Fiesta My car jerks when I drive

Engine Performance problem
2000 Ford Fiesta 4 cyl Front Wheel Drive Manual

High I have a 2000 Ford Fiesta RSi (I'm from South Africa). Anyway, the problem I have with my car is one it doesn't idle correctly sometime is sits on 1000rpm or just below but still not where I know it should be then other times it will slowly climb when I'm stopped, or anytime I put my foot on the clutch, to 2000rpm. Then when I drive and the speed is consistant and the revs are anywhere below 3000rpm the car would start jerking or almost cut out where it starts slowing down and for a second or two doesn't respond when I press the accelerator then suddenly jerks forward. I've replaced the fuel pump already a few times and the problem persists. Thanks