Engine Sputtering above 5k RPM... Please help !!

I am at wits end with this thing. 2006 660 all stock engine/exhaust.

It has developed a bad sputter at about 5k rpm up to the rev limiter. (I have a BRP Tach) It cranks perfect and idles fine. At low speeds it runs perfect. No sputtering until around 5k. I have tried re-jetting and it doesn't change the sputtering at all.

So far I have done :

Installed new OEM COIL and Spark Plug
Removed High Output Stator, Pickup and Rectifier and replaced with Factory parts
Battery shows 12.4v at rest and 13.1 idling
Installed High Volume Fuel Pump
Installed all new fuel lines and new fuel filter
Removed Fuel tank and cleaned
Installed new Pickup tube in tank
Confirmed gas tank vent line is clear and removed anti-rollover valve
Removed Tri-fire CDI and replaced with factory (The sputtering is not as bad with the factory CDI..it is noticeably worse with the Tri-fire unit)
Checked valve clearances
Installed new carburetor
Installed new K&N filter
Grounded green/white wire from reverse switch
Checked compression and it showed 125psi

Nothing I have done has affected the sputtering. I can make the reverse rev limiter kick in by ungrounding the green/white wire from the reverse switch...the sputtering I am dealing with is not the reverse rev limiter. Brake lights, reverse lights..everything works as it should.

Can anybody give me insight as to what to check next? Should I be looking internal engine or focus or carb/electrical issues?

Thanks from Mississippi !