2000 Chevy S-10

Heater problem
2000 Chevy S-10

My truck's gas gauge is messed up. No matter how much gas is in it it says it's completely full. Occasionally while I'm driving, usually after about 250 miles, then the low fuel light will come on. When this happens the needle moves from full to below empty. After a few seconds, or about a minute, it will turn back to full. Any suggestions on how to fix it? Also, it can drive over 300 miles on a single tank, but I have to fill it when it gets to 300 because I am afraid it will run out of gas.