I have a 2004 150 saltwater series mercury outboard. I am having a problem which seems to be common by reading a LOT of posts. I have a bay boat that I take off shore a lot. If conditions are rough, I take it easy going out. If I try to keep my speed at around 20 mph, I have a constant alarm that goes off. Water pumps appears to be working as motor pees just fine. If I shut the motor down and wait for about a minute and a half, alarm is gone when I start it back up. Alarm comes back on within a quarter of a mile. The alarm doesn not sound when I idle or run wide open. Any thoughts?