Heres the issue. I drive a 2005 legacy wagon. base model. 2.5L DOHC. 150,000 miles. The vehicle started overheating so I went through and systematically replaced the entire cooling system. Thermostat, radiator, rad cap, water pump, did timing belt while I was there. The vehicle now will over heat at low speeds after about an hour of use. What I think I have now is a pressure issue. The overflow tank for the coolant overfills, cap pops off and coolant spits out while I am driving. After this occurs the temp gauge starts to vary due to the loss of coolant. I pressure tested the car, no trace of any smoke coming from the exhaust. However the pressure did build up above the cap limit of 16 psi quicker than it should. Also no milky sign in the oil reservoir or on the dipstick. I do not think this is a head gasket issue. Head gaskets were replaced by the previous owner 40,000 miles ago. If anyone has had similar problems with their vehicle and resolved it please contact me, as I am very interested in resolving this re-occurring issue.