Just bought an 04 DeVille with 150,000 miles on it about a month ago and I gotta say I'm in love other than the small overheating issue it sounds like the other two people have. I noticed after about a week of driving it it would do perfectly fine in town with the AC on and everything. but once in a while(usually on really hot days 100+ degrees) after about 5-10min on the highway the temp would either slowly climb to 2 notches above half way then go back down and other times it would continue to overheat. The times it lloked like it would actually overheat I would pull over shut the engine off and let the fan stop running, give it about 5 minutes more to cool then take off. Most times it wont overheat again for the remainder of the drive and other times it happens again and I sit for longer to let it cool. just this past weekend I drained the coolant and changed the thermostat, filled with new DexCool and it seems to actually run better and the 15min highway test I gave it it just stayed at the half way mark except for one time when I first took off, I had the AC on and it went 1 above the halfway mark so I shut the AC off. Then on my way home(about a 10min drive roughly) I kicked the AC back on and it never went above halfway the rest of the way home. I am going to utilize the check list given where it should be a little hotter out today. Im not much of a mechanic on the newer cars but I love this car and I want to make sure its taken care of.