rover 414si- what causes it to jerk?

My Rover 414si car 1996 model started jerking in recent times. the feul filter was changed but no good result. the nozzles were flushed with a carburator cleaner yet nothing good. It jerks when moving and tends to seize power at times. when the car is running at stand still, its rpm is not stable but fluctuates and sometimes seizes eventually. Please, some one should help me out.