Dave I kept having that problem years ago around 5,500 rpm. First thing took out the screaming demon coil pack put the factory one with probably 170k on it that helped I think I could get up to 6k rpm before it started misfire. Tried every spark plug you can think of the iridium made it way worse. Ended up with cheap Autolite copper core 103's I think gapped around 19-22 got the rpms up to 6,750ish.
But I always have a voltage drop in the higher rpms I have 14+ volts at idle and up to around 5k rpms then the voltage starts to drop rapidly down around 11.2 volts at around 6,700 rpm. This in my theory is why the factory coil pack and copper plugs work better in my setup, I'm sure the aftermarket things would excel if I had the voltage to support them, but that's a problem I'm still working on.