doesnt the charging system kinda turn itself off, or stop charging at high revs? If so, when it stops charging, you could be seeing a failing battery? I'm probably wrong, but it sounds real good.
Had the same problem. Was racing one day, and alt light started to come on above 3K or so. Mine would quit chaging altogether, voltage would drop off and everything. Fine if I stayed below 3K or so. I replaced the alt, problem solved. Just my experience.