Gas guage problems

[Richardwa]

71 GMC 2500.
My gas gauge quit working, stayed at 1/2 tank. I unhooked the wire and it went way above full, about the 3 oclock position. I grounded the wire and the gauge went to empty so I figured it was a bad sending unit. When I hooked the wire back up the needle went to between 3/4 and full.
I bought and installed a new sending unit still the same problem. I took the sending unit out and hooked the wire up and grounded it, raise and lower the float and the gauge works as it should. Did the same with the old sending unit and it works as it should also.
I have tried running a ground wire to it and no change. When either sending unit is installed in tank the gauge reads between 3/4 and full.
There is only about 5 gallons of gas in it.
I also unhooked wire from fuse box and gauge went to 3 oclock.
I am running out of ideas here, hope you guys have some ideas.

Sorry for the long post but I wanted to be as specific as possible