Temperture gauge

2002 Buick Rendezvous with 81,000 miles. This question has bugged me from the beginning. The temperture while driving is not up to where normal is when you are sitting idle. The dealer has replaced the thermostat twice but it still does the same thing. I took the thermostat and put it on the stove in water (with a temp gauge in the water) and it worked perfect. Opened at 195 exactly, so I know it's not the thermostat. In the summer time the temperture runs normal, but in the winter when its below 50 degrees it does not run hot enough, except when you stop at a red light. I get 4 miles a gallon better mileage in the summer than in the winter time. Hard to believe, but I have checked it a number of times. I went to the dealership and drove a brand new one and it does the same thing. Buick does not know or they will not tell me why? Do you have any ideas? I think the computer thinks that it is running cold and therefore it sends a signal to increase the gas mixture. Thanks, Mack