My 300TE stays just above 80C now that I changed the thermostat, but when I stop after a drive, the water builds up in the expansion tank, dumps about a tanks worth through the overflow pipe, and then settles back down so that the tank is empty.

Just replaced the t-stat, cap, etc. I cant figure out why it would be doing this. I bled the system as best I could. What would cause the system to swell like that and then drop back down?