1998 renault megane 1.6 fuel starvation problem?

i am having trouble diagnosing a fuel starvation problem with the wife megane. Whenever the fuel level drops below 1/4 of a tank, it seems to judder whist going round corners or up hills as if its running out of fuel.
I know the easy answer is to keep it topped up but its not me thats driving it!!
Any help appreciated as this is really starting to annoy me now(or at lease the wife is!!)

steve
November 2010