'94 940t w/270k. Done a search, got a lot of ideas. Hoping it is not a head gasket.

The temp gauge stays good all the time, right in the middle. During running, especially now that it is hot - the pressure builds within the system to the point that the coolant pushes itself down to what appears visually through the side really low in the reservoir.
After the car has set a spell, I loosen the reservoir cap, and the coolant will rise, almost boiling like - to a higher level within the reservoir. It is not really boiling, just same type behavior.
My first thought was an air pocket somewhere in the system, or a defective t-stat. Over the last few months have installed new heater hoses, heater valve, reservoir cap and t-stat (drilled a hole to let it vent). Radiator is about three years old.

Side issue ..
When the wife drove up today, the radiator fan ran and ran after engine shutoff ...seemed like forever. I have read the faqs about the sensors, and that may be what this is ..just wanted to mention it with all this.

Do I have a problem? Ideas? Thanks for any and all help.