A friend of mine bought an old car that had been sitting for a long time. It had some gas in it and he filled it after he bought it. On the way home about halfway it started to sputter and backfire and lose power. Pulled over to the side of the road and tried to diagnose problem. After about an hour of trying different things it started back with no problems. Drove home and just as he got home started doing the same thing. We couldn't figure it out for a long time until finally we noticed brown dust coming out of the exhaust and on further inspection figured out it was rust.
The gas tank had rust and crud in it and after driving a bit it would all come up off the bottom of tank and get in the fuel. After sitting awhile the crud would settle.and it would run again. Long story short check the tank for crud.
