What is the purpose of the bypass hose

Just thank you're lucky stars it wants to run too cool. My stuff always wants to do otherwise. It's a lot easier to get em to run hotter than cooler.

basically your cooling system is just too efficient on cold days. probably one thing that is happening is that your oil is also not getting up to operating temp either and that can be hard on a motor. you may need to get an electric fan controller/programmer so that you can set the turn-on AND turn-off temps seperately. have the fan come on at 200 degrees and then turn off again at 180-185 degrees.

consider using a different weight oil as well. if you're using a heavier weight oil now switch to a lighter weight and vice-versa and see if that has any effect the overall temps.