rough idle. smooths out with higher rpm.multiple cylinders misfiring. on 2005 Pontiac G6

scanned and came up with p0300 and p0404.performed in line spark test and replaced coil pack with no change.egr valve making faint exaust popping like exaust leak.ruled out crank sensor since it does start.80% sure its egr valve but haven't ruled out cat converter.150,000 miles so either is possible. will the p0404 code determine if converter is plugged?i need help deciding since neither is cheap.keep getting mixed opinions.

Asked by for the 2005 Pontiac G6
Check your Cat first. You should have full flow out of the exhaust pipe. Put your hand close to the end of the exhaust pipe. Have a friend start the car. Rev it up. See if you have full exhaust. If it wont push your hand away, chances are you found the problem. Have you tried tapping on the CAT? When they go into melt down, you will hear what sounds like rocks inside it. Goodluck
thanks for the tip. honestly haven't even messed with the cat because i was going off the p0404 code the scanner read. wouldn't i have noticed poor engine performance before? i had just driven about 50 miles and didn't notice any problem until i parked.i would have thought a plugged cat would be more progressive. thanks again. found the problem, bad egr valve. quick fix for 70 bucks.