My gas gauge is weird... it works when the tank is full and it seems to actually be working with this tank of gas but NORMALLY when I fill up the gauge goes past the full and then as it starts going down it will just drop off... like over to where I cant see it almost. This makes me think its not a grounding issue and its just that the float is bent and I need to straighten it. Ive got a brand new tank and sending unit that my dad installed before the car was mine but I think they're just genaric so that he could get the car running. this is on a 64 dart, and if it make a differnce it is convertible

Any other ideas or does this sound like a plausible reason for this problem?

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64 Convertible Dart GT

Your sending unit uses a wire wound variable reisistor - if the windings have developed a bad spot (very high resistance), then very little current will flow and the gauge will drop to nothing every time you hit that spot.

The windings are very delicate and I've never managed to successfully repair them; if you can get a new sending unit, that's the way to go. Otherwise, J.C.Whitney sells a Datcon inexpensive universal sending unit with the right range.

Theirs calls for you to punch a big hole in the tank and install it from above. That seemed a bad idea to me. On several TMC giveaway cars (http://www.tidewatermoparclub.com/pastcars.html)
I've cobbled together the factory pickup and the Datcon resistor and arm to make a functional sender. The trick is to make sure it all still fits through the hole in the tank when you're done.

definitely the sending unit.To add to what K.Beard said the resistor has a metal contact strip that slides along the resistor giving you your gas reading.It could be losing contact in that area.

Heres a little tip.You do not have to drop your tank to remove the SU.Jack the back of your car up and place two jack stands under the frame.Now you will have plenty of room to remove it.I actually just went through this myself.

Good luck.
JZ