My gauge is doing the exact same thing. I tried bending the float arm to compensate and it just stays reading full longer. I checked wires, grounds, the gauge itself and all is well. I found the following which seems to explain the issue perfectly!
Auto Meter

I replaced my sending unit with one from LMC and it's been like this since. Seems the only solution is to try and find a working OEM one. I've verified the resistance at the different points the article describes and I found what it says to be dead on with my replacement sender.