51 mercury miss while running at 50 to 55 mph

[Brooklynn1]

I have been fighting this miss for awhile, the car runs fine, starts great and has good power when I floor it, runs right up to 70 mph. But running at a steady mph or rpm it has a miss. Carb is stock 885ffg rebuilt, I installed protronic electronic ignition with coil, new plugs cap roter, timing seams to be on. Any thing I missed. Really getting frustrated. Thanks for reading, anybody have any ideas?