is the fuel outlet on the tank on the opposite side from that which the fuel is forced to by the forces generated during cornering?
if it is you have to stop the fuel moving about, you can get fuel tank foam for this job, comes in small cubes that you push down the filler, it absorbs the fuel and stops it sloshing around.
its a bit pricey but is an easy fix, but saves stripping car and buying baffled tank
steve