1986 c-10 surging problem , please help

[dwooten]

I have a 1986 c-10 silverado with a computer controled carb. I had the carb and transmission rebuilt a year ago and engine rebuilt 3 years ago. Recently it has been surging once up to speed after it is in 3rd gear.It feels like maybe the timing is being retarded or something , it runs good for about 3 seconds then bad for 3 seconds, it also has a strong fuel smell when it is surging but I cannot find any fuel leaking any where not sure if it would be the map sensor or maybe something came loose inside the carb ? Any help would be geatly appriciated