gas gauge reading when full but goes down quickly

Update... I jacked the car up and pulled out the sender. It looked good and tested out ok through it full sweep. Approx. 10 to 70 ohms. I did notice however that the plastic float had a bit of gasoline in it so I swapped it out with a brass one I had from a bad sender. Anyhow I hooked it up again while it was sitting on the floor while the wife checked the gauge. It was still not showing correctly so I decided to yank the cluster. I quickly realized I never updated to a solid state IVR on this car so I'll do just that. Hope to have er done in an hour or two.

Update of the update... Ok, so I yanked the dash out, pulled the gas gauge out, modified it so it doesn't put out voltage and installed it back into the dash. I also added a 7805 regulator which I had built up on a small pc board along with a couple of filter caps and a teeny LED to show 5vdc output. Turns out the gauge was bad so I replaced it with the last one I had in stock and it worked just fine. It's now back in the car and the test drive showed the temp. gauge right the first fat mark and fuel gauge reading correctly.