high pressure in cooling system

[jarekt]

I'm wonder if anyone can help diagnose high pressure of gases in cooling system.
Car is running great, still after approx 100 miles on HWY temp is going up.
I can't see any oil in coolant or coolant in oil system. Still there's very high pressure of gases (air?). To be more specific, driving on 5th, 100m/h is not a problem, only as soon as i idle engine, temperature is going up. accelerating will bring tmp down again. When engine stop, there's a lot of spill from overflow tank and i can release a lot of steam from radiator!. When engine is cool, coolant from overflow will go back to radiator.
There's not "white" smoke from pipe, no lost power.
one more: on old engine i have "burbling" noise in core heater. This sound didn't go away when i installed new engine two years ago.

Any ideas?
BTW When it happed first time, i replaced thermostat.