2002 Impala misfiring

Hi,
My girlfriends son's 2002 Impala runs as though it is either starving for fuel or spark, or it cant expell its exhaust.
Its the v6.
Nobody seems to have even noticed this car running this way, so I can't give any more details as to how or when.
It has 105,000 miles. The last time I drove it, (couple of months ago) it ran just fine!
Today I changed the spark plugs and installed a good grade of new wires. No help. I then changed the fuel filter. No help.
Someone suggested that it might be the cat converter. Someone else said that if it was the cat, it would show a code. I get some pressure from the tail pipe.
This morning there was one minor code=something like "periodic misfiring". I cleared it. It did come on one more time, cleared it but has not come on again. No other codes whatsoever.
I am stumped and don't want to throw money at it.
I also unhooked the neg battery cable for an hour. No help
Can anyone help me here?
Thanks, Gary

Sounds like a converter, unbolt it from the manifold then drive it if it seems to be normal after that then you know it was the converter.

I would suggests that you double check all your auto parts -  spark plugs wires, cat converter might needs some replacement. It would also good to have a compression test or a leakdown test. Too little fuel may also cause this problem.

Hope this help and keep us posted.