I have a 2000 merc optimax 135hp that is experiencing dramatic overheating. It started at the end of last season, the boat would overheat at idle and low speeds but would cool down at high speeds or if I revved the engine in neutral. I replaced t-stats and water pump but this did not solve the problem. Before the beginning of this season I replaced the water pump (again) the poppet valve and the spark plugs and I am now experiencing worse overheating at all speeds. The tell tale is strong and the boat does not over heat when hooked up to the hose in front of my house. Most forums suggest that all overheating issues can be solved by one of those 3 things (t-stats, water pump, poppet) so any further suggestions on what it could be would be greatly appreciated.