[undy]

..gut feeling is the t-stat is not seating closed well or those 3 holes drilled in the t-stat are a wee bit larger than 1/16".

Under cooler ambient conditions you may be getting too much coolant flow through those 3 drilled holes or an unseated thermostat, more than the engine's heat output at that time. This holds true especially under low power cruise conditions where there's minimal throttle action. The engine's heat generation is somewhat proportionate to horsepower output. My money's on a defunct thermostat. Let us know what the end result is..