fuel sender reads out of tank, but not in?

Its and 87... last week the guage stopped working and was stuck at 1/4 tank of fuel. I tested the guage and it works fine...goes to full and Empty per the factory service manual. In an effort to avoid having to get one of those airtex senders, I decided to hit the jy for a new sender. Once i got it home, I hooked it all up outside the tank, and moved the float up and down. It worked perfectly so I proceeded to put it back in the tank. Once in, I turned the key on and the guage never moved off "E". I took it out again and turns out it was stuck under the filter. Tested it once more and it worked fine so I put it back in the tank. I had just filled up and had to sifen out about 11 gallons of fuel so I knew roughly what the level was, so once again I turned the key and VIOLA!! It registered the amount that was in the tank. Soooo, I took all 11 gallons and put it back in the tank, went to go and top it off, and as soon as I turned the key the darn guage didnt move...again. I just dont get it!!! Am I just destined to put one of those crappy Airtex sending units in? On the plus side...After two years of procrastination I finally got all my power windows to work

How is the ground wire that's kinda riveted to the fuel module? Loose?

Test the sender with an Ohm meter out side of the tank.
If it doesnt have any bad spots then you know its a problem with the electrical.