1996 silverado running rough

so, come out of the store one day, start the truck up, i can tell right away it is running a litttle rough, so i start towards home, and i can tell its not right. I grew up around cars so i know a little bit about them, not all but some, this feels like it was running on 7 cylinders, not all 8, so it has that small rough when your on the gas or at an idle. going up hills i had to gun it, and i could hear a popping sound, swore it sounded like from under the hood, not the tailpipe. Got it home, checked all the plugs, wires, everything was fine. Swapped out the TPS, not much change. So drove it to the local auto parts store, they hooked up their computer, and came back with no codes. Said everything was fine. on a flat road, moderate load it seems to smooth out and run fine, at a stop or up a hill under heavy load it runs rough. It starts up every time without any issues, i stood by the exhaust while at an idle, you can hear that it is missing or something, rev it up while park doesn't backfire or pop....and runs smooth...Any idea's would greatly be appreciated before i start buying every part, so would like to get it narrowed down. Been reading, some say O2 sensor, clogged CAT, i don't want to rip things apart until i kind of get a idea of what it could be.

Welcome to AF: Need to know engine size, tranny & mileage.

Welcome to AF: Need to know engine size, tranny & mileage.

Engine is a 350, automatic 4x4 with 247,000 miles

pull the valve covers and check to see if the lifters are moving to the right hights. i amd wondering with the hight miles if the cam is wore. have you changed the timeing gears and chain. they streach after a time. every 350 i have had at 150k both problems pop up.