I have a 2005 Sea Doo Islandia that is having issues getting up on plane when you get 5 to 6 adults on board. It planes just fine when when there are 1 to 4 people on board. I checked the impeller and nothing is stuck in there. The RPMs will get up to 6,000 to 6,500 but the boat won't get above 15 mph when there is even a little extra weight. Not sure where to even start to fix this because this can't be normal. No boat shops will even take a look at it anywhere near where I live. Sea Doo gurus, please help!