I have a 1999 DeVille with 156,000 miles on the car. Last winter I had a coolant leak that turned out to be the water pump. I had the water pump and the small belt replaced. That took care of the problem for approx. 6 months. I now have a coolant leak that has no visible signs of the leak. The coolant level is getting worse loosing about 1/2 gal. a day with normal driving habits. The oil does not have any traces of coolant and the coolant reservoir does not have any traces of oil. The exhaust does not smoke or steam. Not sure what to look for, could this be the head gasket? Any help or ideas would be appreciated. Thank You!