That universal sender needs to be customized, if you haven't done so already. You need to adjust it so it is shorter and your float arm is at the precise length. Their specs are down to the 1/8 or 1/16 of an inch if I remember correctly.
Secondly, make sure you have a ground from the top of the sender (outside of tank) going directly to the neg on the battery. I found this to make sure the ground is proper.
Or you could have a faulty gauge, but I suspect ground and float rod length and height
