it is probably back pressure through the exhaust system. If the skis were used in beach settings your ehaust could be filled with sand. it plugs up the exhaust as it heats and makes the ski over heat. Pull the entire exhaust system and make sure it is clear of debris and sand. We had the same thing happening with one of our Yamahas. once the exhaust system was purged their were no furthur problems.
As in all marine engines, they are more sensitive than auto and motor bike engines. check your fuel filters, plugs and the diaphram. all could be contributing.
Running skis full time in a commercial business really lets you know how important maintenance can be. If you are not someone who is just using the skis on holiday you need a real good mechanic. Make friends with him. you will be seeing him alot.
