I think i have bad sending units in my 76, anyone have bouncing fuel guages??

[RyanD.]

I have a 1976 sierra with dual tanks, it has been sitting for 13 years, when both tanks are empty i turn the ignition on, the needle sweeps very quickly to way over full and holds there, when i turn it off it goes back down to half, on tank 2 when i turn the key on it bounces to half and stays there, the same problems happen no matter how much gas is in the tank, has anyone had this problem, i was thinking sending units, i have changed the guage in the dash (old one didnt work at all) to a used one from my parts truck, thanks for any help.