HELP PLEASE!

Hi, i have an 86 ford f-150 with the 300 4.9 and it has had new plugs.. wires.. carb, the whole 9 yards. and it was running great for months... now it still starts and idles very well.. and you could even say it is still runing rather well also... the problem is that when i am going down the road highway speed and let up off the gas for say 5 seconds or so and then lighty give it some gas only to resume speed.. it will act like it's going to stall... it will buck a time or two and then go on.... and i have notice when i go in to pay for gas or something when it is warmed up when i come back out and get back on the road it seems to buck more as i am getting back out on the road.... after it has been turned off! sometimes to the point of making my back tires chirp a bit... push in the cluctch and rev it a few times and go on down the road with no problems till the next time i let off the gas for a few seconds... noticed it does it more in 3rd than in overdrive.. any help would be greatly appreciated!! take care! Tim