Cooling mystery

I have an '02, and I have a cooling mystery I just can't seem to solve. Since the weather cooled, I notice that my temp gauge keeps climbing. The water pump is good - changed 2 years ago. Changed the thermostat to a 180* JET Stat (and it works), Changed upper rad hose/cap assembly in August. I figured I had an air pocket, but repeated burpings only help for a week or so. What I am noticing is the overflow tank keeps filling up, and the coolant in the system is low. I drain the tank, put it back in the system (while burping)... Then a week later, the overflow is full, the system is low! Pressure test on the system showed no leaks; held constant pressure. Any ideas???