Try putting your hand on a carburetor and rev it up to see if there is excess vibration at the carby in the 6-7k rpm range.
Perhaps excess vibration is causing the fuel in the float bowl to become frothy over the 6-7k rpm range?? and affecting power??

If the rubber carby mounts have hardened over time, they may now not be very good at isolating the carby's from engine vibration.
This is just a wild guess. It could easily be something else causing your problem.