I'd look into the ram air system first. there's a bit of plumbing in it and perhaps something has worked it's way loose. There may be ignition issues. The spark plug boots on those engines have a resistor withing that sometimes fails but that usually shows up below 2500 RPM or so.
Another place to look would be the carb diaphragms. They are quite fragile and one may be torn or installed wrong.
Proper operation above 9000 RPM pretty much rules out fuel starvation.
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