I had a similar problem on a 93 c2500. Could not figure it out and had tried everything you name it. It turned out to be the fuel pump. Works great now.

 

Annoyingly can't see it being fuel pump, it also runs on (lpg/liquid petroleum gas) and has the same misfire at idle when running on both fuels despite being a different fuel system/pump.

it has to be sensor/vacuum/electrical connection issue

 

Hello, I know this is an older post. I've got a 94 C1500 5.7 with the same issues and can't seem to fix it for love nor money. Is there any chance you resolved this issue?
Thanks

 

Can you go through the OP’s list and confirm you covered off on what he did? He didn’t the ignition module beneath the distributor cap. Instead of throwing parts at it you might have the garage take a look.

 

Try your manifold pressure sensor. I was on a cross-country trip to CA desert a few months ago when this happened...very rough starting, would barely idle, but ran just fine on the road. My MAP (?) sensor went out...replaced it and the truck was back to normal.

 

Check engine thermostat and make sure it is really warming up. Might not be the gage especially if it moves just a little.