First post here...kinda long so bear with me.

I have a 98 S90 with a recurring coolant problem. The car right now has ~160k miles and I have maintained it exclusively for the past 4 years or so (wifes car and her dad took care of it before that.) It has never had any real issues until now.

The radiator cracked where the upper radiator hose connects so I replaced it about 8 months ago. Everything was fine for 1 week after that then, for those that know the S90 (or maybe other volvo's have this too, I don't know) the little plug in the top right corner of the radiator popped out while driving, shooting all of the coolant out. I checked for obstructions and found that the thermostat had broken up so I replaced that. Filled everything back up, put the plug back in and everything was good until last week (so about 7 months.) The new radiator developed a small leak. Then the plug popped out again and again I lost all coolant. I checked the thermostat again but it was fine. I replaced the radiator again since it had a crack and it has a lifetime warranty. Filled everything up, put that plug back in and all was well until yesterday when that damn plug popped out again.

It's like pressure is building up on one side and follows the path of least resistance blowing that plug out.

So, the system has been opened up twice for radiator replacements and the thermostat has been replaced once yet this stupid plug keeps popping out. Has anyone ever had this happen? What could cause pressure to build up like that? Is there any special procedure for "burping" the coolant system of air that I am missing? Could something like a failed water pump be causing a blockage?

Thanks