Studders at 3000 - 3500 RPM's

Fellow Z riders, I have a problem with my 1983 ZX. When I'm cruising around 3000 rpms the engine studders like it's not getting enough fuel. When I increase gas I don't feel it anymore. If I drive the car very aggressively the problem seems tranparent but in stop in go traffic or city driving I have this problem. Help, I've changed all 6 fuel injectors, fuel filter, air regulator, and tuned up car (plugs, rotor, cap). I was told it may be fuel air mixture causing problem. Does anyone have or had this same problem and what did you do to fix it, other than that the car runs great