Sputtering under load

My Blazer started with misfire the other day. Now, after it runs for a minute or two, when I get into the gas, it starts sputtering, huffing, and making a rattling/popping noise from under the truck. It gets worse if I run the A/C, and has started stalling out when coming to a stop. Figuring that it is a vehicle with almost 180,000 miles on it and still having the original fuel filter, I replaced it. That seems to have helped with the stalling, but the sputtering is still there. I am thinking the catalytic converter is clogged up and possibly that the screen thingy inside of it is coming apart (hence the rattling sound). What do you think?

The fuel pump is less than a year and a half old. The distributor is newer than that. The spark plugs are a few weeks old. I do not know the age of the wires, but they are not really cracked or worn looking and I do not see any sparks jumping when I checked outside in the dark. Check engine light has come and gone a couple of times, but is not currently on, and no codes show on the scanner.