Gas levels & stuff

Ok, my cars gas levels..change. Sometimes when I'm driving it will be on E, and then I'll look up a while later and it's like half way between the E and half full. What the ****? I never know if I'm about to really run out or what because it changes all the time. And no, it's not the obvious - ie. me putting more gas in it or anything like that.

And when it's close to being on empty.. my car acts like it has problems starting. So... what the hell is happenin?