I have a '97 K2500 with the 454. Simply put, it doesn't idle right. When I start it up cold, it revs up to 2000 RPM, slowly settles to idle, and then either abruptly dies or practically dies before roaring right back up and going through it again. In order to keep it on I hold the gas slightly or rev it continuously. It's more likely to stall when cold, but it can happen pretty much anytime it idles. If I let it idle, and then put a small load on the engine (steer or jam brakes or put it into gear) without revving it'll also die, so making K turns is a nightmare.
2nd problem is the afterfire. It makes incredibly loud pops out the tailpipe irregularly - sometimes it'll just keep on popping, sometimes nothing and then a loud bang. The pops don't sound good and regular, but are completely random and never "in sync" with the engine's sound. It's legitimately so loud sometimes it hurts my ears and wakes up my family in the morning. The afterfire is a lot more frequent when cold, but sometimes happens when it's warm. Sometimes holding the engine at 1500 RPM is the only way to get it to stop popping, sometimes that's the only speed at which it'll pop. Sometimes it even feels like the engine is getting robbed of power until the pops stop - I'll put the pedal down, and as soon as the pops stop, the truck gets an extra 'oomph'. Might just be a perfectly timed upshift, but maybe not.

For some background, I replaced the head gaskets, intake gaskets, & exhaust gaskets. After that fix, it didn't run right, and my mechanic adjusted the distributor and tightened the pushrod rocker bolts. After that, it ran great for a few days, until the pops started, and it's been getting worse and worse (might just be winter rolling in?). My tailpipes have holes everywhere, and the flange where the front exhaust pipe meets the rear (under the cab) has a good 1/4" gap. My EGR hose is disconnected - when I took off the intake manifold I had to lift the whole thing up and rotate it to free it, and I forgot to put it back that way. I used to have the hose stuck into the port where it should go and wrapped in aluminum tape, but it didn't hold well and the engine was quite hard to start. So I taped off the EGR hose, as well as the port on the intake, and now it starts better (until it dies ). Also note that I do not have any AIR or smog system.
I've heard elsewhere that exhaust has enough fuel to burn, but due to a lack of oxygen it doesn't, so could it just be the holes? I don't want to get a new set of tailpipes and then have them blow apart because it wasn't the problem.
Any clues as to what might be the issue(s)?
Sorry for the long post, just wanted to make sure I got the whole picture laid out.