Engine trouble

I have a 97 pontiac grand am s.E. With 121000 miles with a 2.4 liter engine I was driving down the freeway and suddenly it lost a lot of power and the check engine light came on I did a diagnostic test with a handheld machine and the code came back po300 which indicated a misfire well the car would still run just not idle and ran with a shudder I replaced the plugs and the boots and the coil pack and the car then ran a little better and would idle even but ran with a chugging kind of sound and the readout still read misfire even after I had reset the computer to where it says pass but once I run it for about 30 seconds the light comes on and then says a misfire again also the first time this happened the cat converter turned cherry red now it runs as if it is being choked to death but will idle and the exhaust smells heavily of gas any suggestions could it have jumped time?