My friend has a 1998 Mercury 225 Offshore that overheats and he has recently replaced the thermostat and impeller, but the problem still persists.<br /><br />Here are the symptons: After running 10 minutes or so, the engine overheats, alarm sounds. He tests the pee temperature by hand and the water is too hot to the touch. He kills the engine for only 20 seconds, restarts and the water is warm to the touch. The pee flow is good, then sometimes it's not. He can put the engine in neutral, rev up and the pee flow is good. When he returns to below revved engine speed, the flow remains normal most times, but often (not consistently) turns to a dribble. A mechanic friend and himself recently replaced the impeller and thermostat (repairs were not done by a pro). Any suggestions for him? 
