There are several things this could possibly be. I am running almost the same setup. The Yamaha VMax HPDI is an awesome motor. It has several features that you may not know about. #1 If you bump the governor at 6000 RPM's, it is designed to reduce your RPM's to keep from blowing up. All you have to do is ease off the gas a little and you will feel the RPM's kick back in. #2 If your over heat alarm or low oil reservoir alarm is not working, your engine may be over heating, in return, it does the same think similar to bumping the RPM governor. It will reduce RPM's until it cools. These may not be the problem, but if these are not the problem, UH OH!