This one defies (my) logic! Is there something I'm missing?

We are running twin Yamaha 200s on an 8 mtr twin hull in The Gulf with
a water temperatue of 30 degrees C. This problem has only developed
over the last 5 months.

The problem is that the buzzer on the *left* engine comes on. The boat
yard has checked *everything* including the thermostat, and don't know
what's wrong!! Impellers have been changed during the last 12 months.

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First of all, when the left engine buzzer comes on, the engine
physically vibrates. If neither control is touched at all, the speed
of the boat slowly drops off (from 20 knots to about 15 knots); then
the warning stops, the vibration immediately stops, and the speed
slowly picks up to 20 again, all without human intervention.

My deduction is that when the warning starts, the electonics
introduces a 'mis firing' into the ignition system as a fail-safe
method of reducing revs and protecting the engine.

So be it.

Situation 1:
Left engine 70%; Right engine 90%; speed 19 knots. Happy for half an
hour.

Increase *right* engine to 100%; speed picks up to 23 knots, and after
2 minutes, the warning on the *left* engine comes on. Reduce power on
right engine, and alarm stops. Left engine has not been touched at
all.

First thought:
The wires are crossed... it's the wrong engine that is giving the
warning!!! That seems impossible and does not fit the next situation.

Situation 2:
Left engine 70%; Right engine 90%; speed 19 knots. Happy.
Increase *left* engine to 80%; speed picks up to 23 knots. *Left*
engine warning comes on after a few minutes (as you would expect!)

It almost suggests that too much physical speed prevents effective
cooling of the left engine... that sounds impossible!! (but then, all
logical explanations sound impossible!)

To state the obvious... this whole thing is crazy!! Diagnosis is
obviously not helped by the lack of rev counters. We are getting them
replaced.

How about the suggestion that the engine 'knows' that the rev counter
is not working, and won't got over a certain speed to protect
itself??? Well, give me marks for creativity!

Any logical (or Illogical!) suggestions?