Check your regulator. A discharged battery should be charged while sitting on a running machine but if it isn't charged, the symptoms are exactly as you describe them:
The lower the voltage on the battery gets, the lower your rpms will be, as the bike uses more current at higher rpms (more ignitions pr minute).

After a rebuild I forgot to connect the regulator and my max. rpms would go lower and lower as the battery became discharged.

If yours stayed at 8000rpms, then maybe its charging a little, but not enough.
_________________
Poul
"If Life Gets Boring, Risk It!"
MC21SE, KISS box
MC21SP, HRC box, racing loom