SSEi sputter

I have a '92 SSEi and sometimes I lose power and sometimes I don't. It seems like when I have the problem, my boost guage goes wacky, and my idle jumps around...kind of like it's missing in one cylinder. I had an engine code a while back, and replaced the cam sensor. That helped but it has never ran right after that either. No more engine codes though. Also, I don't know if this is a seperate problem or could be the cause of my whole problem....but when the gas gauge reads half tank, I only have about 3 gallons left. And when I take a hard right, it runs out of gas...the lower it gets the less of a turn you need to take to get it to do it. I'm thinking that's something to do with the sending unit and pump inside the tank, but don't know if that would cause my sputtering problem. With the sputtering, most of the time when you get heavy on the gas, there's no power, it doesn't downshift, and sometimes you can hear it knock a little. It doesn't do all this all the time....sometimes you get on it and it takes off like new. What could it be?