I've been trouble shooting this for the last couple months so please review what I replaced-inspected to this point in time.

Symptom: When the boat-engine is been sitting for a couple of days and I take it out on the water it runs like a "Champ" it is good and strong.
After about 20 -30 minutes or so, an Alarm comes on and she loses power and returns to idle and alarm shuts off. If I go to "Throttle up" again she gets up on plane (2000 Rpm) the "Alarm sounds" and she loses power return to idle or stalls. But the engine starts back up every time. I can get back to port in Limp mode (1100 rpm).
I have changed the Impeller, Thermostats, Poppet valve is completely rebuilt; Tell Tale water discharge and pressure gauge (25psi) is good. Fuel-Water filter replaced. Oil pump gears has been visually inspected, (Float Replaced) in cowl reservoir. All appears functional.

I found and replaced a bad Voltage rectifier (Top half) unit was bad. This might of induced this problem to begin with.
For testing purposes I pulled port side temperature sensor and I noticed it does shut down engine when disconnected. I have replacement temperature (Overheat sensor) in hand and I will replace today.

If this does not work, does anybody have any suggestions in what to change out next: It is going to get expensive from here so I need to be precise.
1. Alarm Control Module $278.00
2. Control Module Ignition $267.00
Please advise and Thank you in advance...
CJ

By early indicators this is not the problem. Mercury Tech manual states if resistance changes sharply then it is not the sensor. In which it did,,when I dropped it in Ice water. 900 Ohms (Room temp) increases resistance when colder, decreases when warmer 400 ohms. 110 Fahrenheit

Tech manual says Starboard side Head temp sensor is for overheat Horn-Alarm. it is either this sensor or I need to go on to Control Modules IGN / Detonator.
the story continues and stay tuned ....

Well it is fixed. Running strong again and reliable. I replaced the Control Module -IGN.... This has taken care of the problem. This Module controls: Oil Alarm, Rev limiter, timing, engine shut down, idle control, stabilizer etc.. Hopefully I should be good to go for at least the next several seasons. Peace Out..