Side to side baffles dont take care of fore to aft movement of the fluid. Do you have a sender that has the float on an arm? Perhaps you can reorient the arm 90 degrees to reduce the effect of the fluid movement (assuming that the problem is occurring due to slosh of the gas).

You probably cant use fuel cell foam to reduce the slosh because it will interfere with the moving pickup. When we use fuel cell foam in a tank, we use an inductive fuel level sensor.

Here is an inductive pickup that works well:

http://www.centroidproducts.com/

We have used two of them and they work great off road where there is a lot of slosh.

Mike in Tucson