My husband and I have a Merc Force 125 that has alluded mechanics. We use it to troll (with a bucket in tow). After several hours, the warn horn goes off indicating overheating. So far, we have flushed the ports, replaced the water pump and impellor (twice) and removed the thermostat. Now once again, during the peak salmon fishing season - it is back in the shop. Has anyone ever experienced this and what was the verdict?