Islandia won't stay on plane

[philinkeys]

Problem with SeaDoo Islandia with a 240 Mercury engine.
While on plane the motor would go to idle speed and will not go back on plane until engine cools down. When this happens the boat won't go over 5 mph.
Does anyone have an idea what is causing this to happen

 

[stephensimmons]

Over heating problem? Did you check for blockage in the cooling system? Maybe thermostat's if it has any?