I have a o3 plate st170, i have an intermittant problem, when you hit 3000rpm there is nothing and car judders and kangaroos and will not go, this is in every gear around the same rpm,the engine management light is on iv had the diagnostic on it and it came up pre cat lambda sensor, i replaced it and had the fault reset, the light then came back????reading up on internet sounds like fuel pump/filter but have already thrown alot of money into car any help would be greately appreciated