I have a 1985 GMC C1500 Pickup. It has a Chevy

I have a 1985 GMC C1500 Pickup. It has a Chevy 305 with a Rochester Quadra-Jet Carb. The problem is when it is cold, the truck starts up just fine, idles fine and has power. After the truck warms up in about 2 or 3 miles, it jerks and seems to misfire at driving speed. It has no power unless I punch it to the floor. Now it will kill at stop signs or if it doesn't, it will idle so low that it sounds like an old tractor. Adjusting the idle does not help. At take off, it stumbles and shakes a little. Feels like I am pulling a semi trailer with it. I have done all the routine stuff...ie spark plugs, wires, rotor, fuel filter, air filter, vac lines, timing, and wiring the choke open (thought it might have been automatic choke was faulty. So I unplugged it and wired the butterfly open.) One more thing..the fuel consumption is unreal....like 5-6 MPG...Anyone have any ideas?