If you can reach the bellows that holds the carbon ring against the stainless steel "washer" which is attached to the driveshaft, try to pull it backwards towards the stern (grasping the front of the bellows and pulling it backwards with all your might by hand!).... if you can pull that bellows backwards and open up a gap, then the bellows is not adjusted correctly and at higher speeds it's opening up and sucking air into the pump thru the carbon ring seal! It should be VERY hard to pull the bellows back if you can even do it by hand! If it's not tight agains the SS ring on the driveshaft, that's a problem right there.
That's my only thought on this one. Good luck and let us know what you find! It's NOT key or programming related though you are making good rpm's just not proper top speed a definite indicator of a pump problem!
- Michael