1991 lincoln town car fuel gauge

I have a magnetic fuel gauge with the needle. when i turn the key on, the gauge goes from full to 3/4, back to full. it is buried on full at all other times. i replaced the slosh module, with no change. what can i do to fix it. two ford parts guys said it's a wire ground somewhere. i have electric to and from the gas tank sending unit. i checked the battery and alternater connections and they are clean.because the needle moves , i assume the gauge is ok. i ran a wire from s to g, where the slosh module plugs in, and the gauge went to empty with the low fuel light coming on.