2012 210SLX Mercruiser MAg 350 Engine Temperature Gauge Issue

[Jeff Skippon]

When started and left in neutral at idle for a few minutes the engine temperature gauge reflects an accurate temperature of 160 to 165 degrees. within two minutes of being underway at any speed above idle the engine temperature gauge drops down to 70 to 80 degrees. Return to neutral at idle and within a few minutes engine temp returns to normal at 160 to 165. Have changed water pump, thermostat and temperature sensor and the condition persists. Engine runs fine even when the temperature gauge is indicating a low temperature.
Mechanic and I cannot figure it out. Anybody ever encounter this before? Any suggestions for a solution?