Fuel sender going bad?

I'm not %100 sure if this is the problem or if I have a grounding issue; My '74 swinger's fuel gauge has been acting odd, it's acting like the tank won't fill all the way, but I know it is. the gauge used to go a little past the full line when full, now it won't reach the full line. When filling, it also takes less fuel than it should.

My question is; could my fuel sender (a factory unit that tested fine when I got it) be going bad, Or could I have a grounding issue somewhere? Which is the more likely of the two or is something else more likely?

If it is the sender, who makes a good factory style replacement?