About two weeks ago, out of the blue, my '98 4.3 started acting up. Right when I start it, it runs perfect. About 30-60 seconds in, there's hesitation if you hit the throttle. If you put it under load (1500-2500) it sputters and sounds like its misfiring. It even backfires out the exhaust (not through the TB). After 3k, it starts to sound somewhat normal again.
I've done plugs, wires, distributor cap & rotor, MAF, ICM, coil, fuel pressure check (60), swapped the fuel pump for another one I have laying around, fuel filter, and engine temp sensor. Same results.
Something I did notice over the past few weeks prior to this is that my exhaust has been getting hot. I have air lines going from my AC that's converted in to a compressor to a tank I have in the rear and while I never had an issue before, the air line closest to the exhaust (6-8" away) actually melted. Muffler and tail section is new, but has an old cat on it.
At this point, I'm leaning spider injector, regulator, cat, or EGR, but thought I would check with the pros first.
I was thinking a simple cat check by disconnecting the exhaust at the collector to see if theres any change. I wouldn't need to run it long like that to know so there should be minimal risk.
But, I don't see how a cold cat would behave differently from a hot cat if its clogged?? So, I'm leaning towards spider injector or regulator, which is what I've been avoiding due to the hassle of tearing it apart. But, again, looking to the pros on this one.

Thanks for any advice you have!

 

My truck did this also, spent a couple of hundred to figure out it was a loose spark plug

 

Well, I decided if I was going to check all the plugs, I would throw a brand new set of AC Delco's in. No change. Definitely not related to spark plugs.