All of a sudden I have a problem with overheating or almost overheating. Every time I'm in stop and go traffic or go up a big hill, the temp gauge climbs right up. I even had the check gauge light come on once!! The only way to get it to come down is to crank the heat and it'll go back down to normal after a few minutes of driving. I was thinking I'd start with a new thermostat? Any ideas or should I just have the radiator flushed and fluid changed? D has 160,000 miles on it and I've never had it flushed or drained. Any help would be great.