Need help with Fuel Gauge 12V conversion

I converted my 53 Chevy to 12V a few years ago. Did everything right. Kept the original dash gauges, and used the 12v to 6v reduction resistors on them. Oil pressure and temp work fine. Fuel gauge, however, which worked fine before, is acting silly. Car off, goes to E as normal. Turn the ignition on, it goes all the way up to F. Start the car, it goes back down to E. Have about a half a tank of gas. Any ideas?
-CP
http://53chevyhotrod.com/images/chevy1.jpg

sounds like a dead sender unit in the tank. Get under there and disconnect the wire to the sender and see wich way it goes (probally empty) now ground that wire and see if it goes to full (or vice versus) If it does replace the sender in the tank...

Thanks, I'll try that this weekend. Very well might be, since it's the original one from 53
-CP