ljking2

I have a 14' single engine 1996 Sea Doo Challenger. It works great on Lake Erie where we bought it, but back home in Colorado it struggles with the altitude. We have re-jetted the carbs and just recently put in a high altitude impeller. When running premium gas in it it will work on Chatfield Lake at roughly 5400', but when we take it to our favorite lake in the mountains, Lake Dillon at roughly 9000' it still has issues. When you first pull away from the dock it will spool up to 6500 RPM and plane fine, but after 30 sec. to 1 min the RPM abruptly drops to 5500 and once you drop out of plane you're pushing your bow wave at 10 MPH the whole way home. What could cause this? There is no vacuum in the gas tank (I have lostened the gas cap to make sure) and I have run enough fuel additive through it to choke an elephant so I don't think there is anything in the fuel system. I suspect something electronic due to how quick the change is. Is there an O2 sensor, a heat sensor, a pressure sensor in the cooling system, anything that could cause this? Any help would be appreciated. Thanks! Lance