What's "holding high rpms?" More than 6300? More than 6800?
Since you hit 6300 in first and second, you should still be able to hit 6300 in third;and Ima guessing you're not triggering the MSD with points, so I doubt this is an ignition system issue.
Since it occurs under "severe" load, IMO, it has to be fuel or plugs. I'm leaning towards overheating plugs or possibly detonation.
Install new colder plugs,upjet both sides 2 numbers,and back off the power-timing 4 degrees; then try again.
If that cures it, go back to tuning.
If it doesn't cure it; IDK.
