rough running

Hi all,
Can anyone help me? I have an 1800 GTI cab on a 1989 plate. I have a running battle with rough idle and running, but lately ive noticed if the car is nearly out of fuel it runs perfectly. As soon as I put even ?10 of fuel in it I get rough lumpy acceleration again.
Any good advice out there?
Thanks
Bez

sounds like the petrol tank needs cleaning.

also worth popping out the lift pump and checking the filter.