I noticed last night when I was driving with the heat on, the car would not completely warm up. The temp gauge is usually around the half way point and with the heat on it only got to about a quarter of the way. I turned the heat off and it got to the normal temp. It was probably about 50 or 60 degrees outside, not too cold.

What might cause this?

This is a 94 850

 

Check the thermostat and make sure it is not broke.

 

Is it normal for the thermostat to break in these cars?

 

Pretty common.