90 Wave Jammer issue

I have a 1990 Yamaha Wave Jammer 500cc. When I put it in the water, it will start up fine, and run great for about 15 minutes. Then it will slowly lose RPM's and in about 10 seconds all it will do is idle. If I give it throttle, it will die. Now if I start it back up it will run great for about 30 seconds to sometimes a couple minutes, and then same thing again, slowly lose RPM's and just idle, give it throttle and she dies. I am all out of ideas, so now I ask for your inputs. Thank you.