First thing I would check is engine speed. get a laser tach and check the engine RPM and compare it to fast idle spec. I know some of them can be set way too low. made a world of difference on mine
Make sure both sides drive at the same speed(drives in a straight line). If they are different, it could indicate a problem.
Next thing to check would be engine power. Does it seem to "bog" alot? Butt the bucket up against a solid object and try to spin the tires. Does it spin them easily? If no, you have are possibly low on engine power.
Also make sure the fuel filter is new. Check all those things and let us know.