HELP SOS 351 C pops back and stalls.

Can anyone point me in the right direction? I know, it's not a Shelby, but I need somthing to drive while I'm finishing up my 67 GT 500. I bought a 71 Torino GT convertible with a 351 Cleveland 2 barrel. The factory carb was professionally rebuilt rebuilt before I bought it. Both hot and cold, the carb would pop back and sometimes stall. I checked everything I could think of. Then I swapped carbs with a GT/CS I own. Both are Ford 2100's. The problem seemed to go away. I bought a Holley rebuilt (just until I finish my Baby) and 95% of the problem went away. Still a flat spot when it's cold. Where should I start???