Engine alarm : 1998 scr3200, 5.7 merc w/ bravo 2

[SummerBreeze]

In the past when I've had an engine alarm, I've been able to track down the problem within 5 minutes. This one has me baffled.

Engine alarm on starboared motor goes on intermittently at 2000 rpms...and on steady above 2500rpms, but below 2000 rpms, no engine alarm.

1. Removed oil sender and pressure sensor leads, continueed alarm.
2. Removed gear lube oil sensor lead, continuued alarm.
3. Removed water temp sensor lead, continuued alarm.

Two other interesting, but possibly unrelated issues...The battery is a little down on that side, I noticeted, labored starting on that motor. When the alarm goes kicks on, I notice a drop in rpms by anywhere from 200-400 rpms.
And the only thing which has changed since the last time the boat was performing properly was that i installed a new seawater pump. Steady and consistent water temp reading since that install of between 150-175 degrees.

Ideas on the alarm issue ??? I'm at a loss.

[SummerBreeze]

Long story short....turns out to be ECT sensor.

[SummerBreeze]

Turned out to be ECT sensor on the right side of the thermostat.

Here's the confusion...When you remove that sensor lead, you are still reading water temp at the helm, so I thought that this sensor had nothing to do with anything. The sensor on the opposite side of the the thermostat, when unplugged, stopped sending info to the helm gauge, so I thought that was the only sensor associated with the engine alarm.

Sure enough, once I removed the ECT sensor lead and revved the engine up to 3000 rpm, no engine alarm. Now the question becomes, is it simply a bad sensor, or what other anomaly (slightly blocked thermostat) could be causing the alarm to kick on. Even when the alarm comes on, the engine temp is at a very safe 175 degrees, so it doesn't seem to have caused any stress on the motor...

Your thoughts are welcomed...