yet another fuel gauge thread - reads full all the time

1970 Dart with a standard (not rallye) dash

So I installed a new fuel sending unit in my tank. I got it set the way I wanted it by bending the arm and it worked fine for a couple months, though it wasn't driven much, and now it reads full all the time (like on the F). The car has a new style IVF and an original style gauge that was tested before installation. New circuit boards and new dash wiring harness (M&H) within the last year or two. The temp gauge works fine and is on the same IVF. The sending unit has a solid wired ground to the frame.

My best guess is the sending unit is defective, but wanted to see what other ideas folks might have before going through the whole, drain the tank and replace the unit thing...