Sounds like a simple bad thermostat. Shouldn't take an overnight visit. Simply on a relatively cold day idle the car until the radiator fans cycle on. Then using a scan tool note the ECT (engine coolant temp sensor) value. If during the road test the value drops below 180F'ish, its bad.Last winter ('02) I experienced the frustration of higher fuel consumtion in the middle of driving - after the car had 'warmed up' - because the engine could not keep itself warm when the weather dropped below 35-40F or so. Within a minute or so of coasting (probably fuel stop mode) I could watch the temperature bar graph drop to 1/4 or 1/8 - usually it's just below half.
John K. Bullock
aka. Insightful Trekker