At 6000 rpm it feels like the boat noses over and the rpms go up. Could this be cavitation at this point?
I would think so.
What loader do you have?
You did not say if you had a droop or place diverter, I would recomend both.
From the pic I see 2 cables so I assume it has a place diverter, Most seem to run best with around a 3-6 foot tail (that is just a rough guess you will have to find what works best for you). In the pic it looks like the diverter is down to low in my opinion.