I am having trouble with the fuel gauge on my 2000 Chevy Venture. It has a mind of it's own!

The only time it reads accurately is when you fill it up. It sometimes even behaves like the speedometer, going higher when you accelerate. It has me puzzled. We also no longer enjoy the same mileage and power that we did when we purchased it second hand last year. I have been told that it could be from using Shell fuel in the past (additive problem for GMs). Is this possible? Any suggestions/advice out there?