'91 champ to chump!

I own a 91 caprice with an original 305 , with only 44,000 miles. For the past 5 months i have been experiancing a problem that i have never before encountered in an automobile, when i first start the engine "COLD" there wiLL be a very slight pop out the exhaust like a small backfire, and then the engine runs fine for about 10 minutes. once the engine is warm aLL i hear is "POP-POP-POPITY-POP" the car jerks and stutters aLL the time and barely wants to run at this point, if i stop at a red light and goose the throttle the engine dies, but restarts "KNOCK ON WOOD" fine and then it goes back to POPITY POP. untiL i park it for about 6 hours. this car used to run like a champ now its just a chump. does anyone out there have any clue as to what is wrong? guesses= catalytic converter,,,,or oxygen sensor,,,,,, bad gas?

I'd start with the simple things first. Check your spark plug wires, cap and rotor for any abnormalities.

Try unplugging the vacuum line for the EGR. Plug the line with a golf tee. Go for a drive. If symptoms stop, I'd say that's your problem. With the Egr unplugged though, you may get some spark knock when the engine is hot.

Is the car throwing any codes?

Thanks for the input guys! I hope i can get her running like a champ again...