I just think your panel is trying to tell you its ready "for a long winters nap"

 

John, I also have vortecs on my sbc. I tried the hole in the thermostat and had similar problems in cold weather. I only had one hole in mine and it didn't get hot enough. I'm also using a vintage air unit, but with a serpentine belt set up and the associated water pump. My solution was to put a standard thermostat in it, no holes. I bypassed the thermostat by running a 5/8" heater hose from the intake to the suction side of the water pump. I have a T in that line for the heater return, also. The pump only circulates water within the engine and heater until the thermostat opens. Before I installed my current radiator, I had the heater return running to a bung on the back of the radiator and the bypass from the intake to the pump. In cold weather it would not get hot enough, either. The water only made one pass through the hot engine, into the heater then directly to the radiator, bypassing the thermostat. I imagine that on really cold days with a large cooling system, the thermostat is staying closed more than we think.
Rusty