part throttle bog/hessitation/backfire

hey guys. i had my 383 dart out the other day for a short drive. it has developed a wierd problem. this is all new and all of a sudden. at cruise speeds 2000-2500 rpm it runs great. then at idle its perfect. at WOT runs like a raped ape. i was cruising in high gear. 2700 rpm. give it about 1/3-1/2 throttle and she runs super rough, pops and sputters and loosed power. let off and it goes away. floor it and it goes away. it sat for few months in cold weather in my non heated, non insulated garage. could the **** gas be gumming up the carb? it kinda feels like it could be timing related. i didnt have time to pull the dizzy apart but i wonder if its got moisture inside the cap and getting spark scatter.

specs.

383 stock bottom end
m1 inkate
pro form 750
254 @ .050 .525 lift with 1.5 ratio roller tip rockers
tti 1 7/8 headers
4.10 rear gear 28" tire
4500 stall

where would you guys start?