engine miss at highway speeds

[nvrforgotten52]

hi,
i have a 69 c10 longbed with a 250 ci motor and a th350 tranny, and i have a bad miss at speeds above 55 mph. i can accelerate great up to 55 but when i try to go faster than that the engine feels like it dies but when i let up on the accelerator the engine feels like it turns back on. it has a rebuild carb, brand new head with hardened valve seats, new air cleaner, no vaccum leaks, free flowing exaust. its stumped the crap out of me so far any suggestions?
thanks,
david