I've been having a problem with my 96 vortec . At highway speeds (65-70) and with a light throttle it starts to misfire and jerk a little. The rpms dont move they stay still , and i hear a little popping or backfiring. I changed the cap-rotor-plugs and wires and still the same result. I also noticed that my catalytic converter went bad . Its rattling and makes a lot of noise. Any suggestions?