I have a 2013 Suburban with the small V8 engine. Normal engine temp puts the gage right about on the 210 mark if I can get it there. In cold weather I cant even get to the 210 mark unless I get on the interstate and run it up to 65 mph. Then it will warm up. The other day I went up a hill to pick up a bag of ear corn and it did warm up to 210 going up the hill but coming back the gage dropped back to the half way mark and never got back to 210. The dealer put a new thermostat in it about a week ago but it still keeps doing this. In real cold weather it never gets past the half way mark no matter how far you drive. What else could cause this problem since the thermostat has already been replaced?