I had a similar problem with a 2002 6X4 Gator I bought last year.. I am new here so take what I post for what it is worth. It looked like the previous owner "serviced" it just before he put a FOR SALE sign on it. It had new oil new oil filter, new fuel filter, new spark plugs (wrong heat range - 3 ranges too cold), new anti-freeze, etc. My engine would over heat after 15 minutes or so. After many months I found air in the cooling system. The service manual has a section on refilling the cooling system and how to bleed the system to get ALL the air out of it. Right behind the carb in the intake manifold is a bolt with a BLACK head on the bolt. On the Gator I bought the BLACK had come off the bolt. After replacing the coolant you MUST remove this bolt and run the engine and add new coolant into the cooling system through the bolt hole into the intake. When you cannot add any more coolant through the bolt hole loosely replace this bolt and run the engine until ALL the air is out of the system by noticing NO air bubbles coming out around the bolt threads. Then tighten the bolt. This was my problem. Even though the cooling system seemed to work properly, the air in the system caused the engine to over heat after running for 15 minutes or so. I hope this helps.