Yamaha 50hp 4-Stroke alarm

I have a 1999 Yamaha 50hp 4-stroke with only 280 hrs on it. I change the oil and plugs each season. The water pump was replaced last year with what is supposed to be Yamaha's 'improved' impeller. It ran fine the rest of last summer. This summer a steady alarm sounds at low speed then shuts down the engine rpm, but not off. The oil and overheat lights always stay off. There is a good stream of barely warm water flowing out the back of the engine. When I turn off the key and allow the engine to stop then turn it back on the alarm will stay off until I restart. At low speed (pulling away from shore, no wake zones, etc.) it usually goes back on and the rpms drop off again. If I can get it up to 4000 rpm or more before the alarm goes back on, it tends to stay off until I go back down to low speed. My local Yamaha sales and mechanic appears lost. I would appreciate your help.

[boats.net]

Probably need to check oil pressure, oil sensor and overheat sensor. Most of the time it's a bad water pump housing.