350 running rough

[ya it is a classic]

hey guys i got a 72 gmc 1/2 ton 350 4 speed. the motors got 20,000 since a complete rebuild. its got a quadrajet and i put some headers on it (stock headers off something?) anyways its my DD 30 miles a day. i run scrap 75 miles away every weekend and cruse alot. i wrap the engine up often and sometimes run open headers if im tweaking my exhaust. i have purple hornies 18in header mufflers with about 4.5 feet of 3in with turn downs.
now my problems started when i got the pickup last summer. if it was 60 degrees out it would hardly start, i mean it would start but wouldnt like to keep running. and if it got any colder then that it sucked. anyways i finally acted like i knew what i was doing and adjusted the 2 air fuel? screws. i finally got it dialed down and it ran great for awhile. then one night it was -10 outside and i started it up and it ran on like 4 cylinders for about 10 minutes then it got warmed up and started running right. its done that a couple more times when its been about 15 degrees outside and i cold start it. so anyways the pickup was running great and i was in a town about 20 miles away and i bought a magazine and sat in my pickup idling reading the magazine for about 20 minutes then i drove home. i was on the interstate and the pickup kinda jerked twice on the 20 miles drive. almost like the engine stopped running for half a second. that was about 5 days ago and its got to it will hardly run. i thought it might have been running rich cause it was popping out the exhaust when i was just driving normally, and doing that half second stop alot. anyways i pulled a plug and the (i forget what its called but its not the electrode tip, its what it arcs to) anyways that was almost white(the plugs have about 1000 miles on them) so i turned the 2 air fuel screws out but it didnt much better, its drivableish now. and i got a carb kit that my neighbors going to help me put in so mabey tomorrow. the only thing is that i gassed up in the other town so mabey there was something in teh fuel and mabey plugged the filter? ive gassed up since then. anyways im going to the shop and hopefully ill get it fixed. but any input anyone has would be greatly appreciated! thanks