2003 Cavalier fuel gauge problem

About a month ago I purchased a 2003 Cavalier for my daughter with about 75,000 miles on it. At first all seemed in very good condition but in the last week one problem has occured a couple of times. The fuel gage has started to read way over full top to the point that it interfers with the temp guage pointer. When starting the car all the gauges go to full reverse and then go to their proper position. When this first happened I though that is might be a sulpher build-up on the float and I added a can of Teckno to the tank to see if it would fix it. Problem got better in a couple of days and I thought "Lucked out here". But this morning it returned. I put the car on ramps and checked out the wires both for clean contacts and also to see if any might be broken by pulling on the connections. Nothing helped. I thought maybe the float or internal connection were bad and that I would have to pull the tank. Here is where it gets odd. I started the car in preparation for getting off the ramps. It seemed to me that the gauge wasn't as far out as it was at first. So I did it again. Better. Again, better. After 8 starts and short runs the guage then read about where I expected it should with the amount of gas in it. Does anyone hae any ideas about what might cause this or experenced it in your car? Perhaps a guage in the dash problem? Any advice appreciated. Thanks.