SpitnCobra

ok my problem is kinda hard to describe, buthere it goes. My car would run just fine cold for about8 miles or so then all of a sudden it starts acting like it's missfiring, really bad. Even at a complete stop it still does it. The RPM needle be going up & down like bumping really fast. But i know it's not a misfire. This guy hooked it up to his meter & it showed left lean bank going bad (I think it was 91 or 92 can't really remember) but I changed the 02, fuel pressure regulator, temp sensor& the injectors r good. What is changing from when the car is cold to when it reaches operating temp. sorry by the way it's a 1993 Mustang GT 302 stock 82,000 miles