Final update (hopefully): It looks like the thermostat they initially installed was bad. I brought it back in and they swapped out a new one and everything appears to be working normal. I test drove it this evening at sustained speeds of over 40 mph with the ambient temperature at 34 degrees F. I won't be entirely confident until I get it out on the road in some weather with ambient temperatures lower than 20 degrees F but it looks like I'm back in action.
Again, thank you all for the assistance. Your discussions helped put me in a better position to talk with the technician and ensure I was getting the correct level of service.