This was like reading a really good mystery, just like you had stated in your thread name. But it's still missing the climatic ending! -
Did you ever discover the reason for the power loss?
One thought not mentioned so far: is the carb perfectly vertical? - if not the float could be unbalanced, and closing off the fuel prematurely, thus starving the motor. You may have a full fuel bowl when starting off, and then it may provide only intermittent fuel as you ride due to vibration.
It's something simple, but vital to have this right.
I have a Powerking as well, so I have a vested interest in what you discovered.
Since this is my first post on this site, figured I should toss up a pic to validate my credibility