should I replace everything?
 

Check engine light comes on sporadically...I checked it once while it was on and it said o2 sensor and catalytic converter needed work...light seems to go away whenever i put in brand name gas like shell or chevron...any ideas?

Use good gasoline.

Actually my first idea was to find out what kind of car we're talking about. 1977 Accord? Carburator or fuel-inj?