Gas gauge problems are usually related to one of two causes; the float isn't working right because it's flooded or soaked with gas or the ground isn't working well.
You say you replaced the sending unit. That should have included the float, so I would look for grounding problems.
One way to test the gauge is to remove it from the tank, ground the flange properly and then move the float arm up and down. The gauge should respond properly. If it does, the problem is likely the ground on the tank. One way to solve the problem is to run a ground wire from one of the flange mounting screws to a good ground on the frame.
