Its the transition from the idle to enrichment circuit of the carb, happens at about 2000 RPM...coincidence? Check your high speed (inboard) air bleeds on a Holley. shoot some carb cleaner in there, or even a thin wire if you got one. Also on a vacuum secondary, check your vacuum diaphragm system. It relies on intake air passing a port in the venturi to create a vacuum on top of the vacuum pot to slowly open the secondaries. If the spring is too soft, it will open early and cause a bog. If its too strong, the secondaries wont open properly and will cause a flat spot in acceleration. This passage is sealed by a small cork O-ring that can go unnoticed in a rebuild.