2002 Peugeot 406 Air in coolant

I have had to top up my coolant twice in 3 weeks. On closer inspection, the coolant is being forced out of the cap of the expansion tank (which is firmly screwed on, seal in tact). After the engine is run, I have bled to system a couple of times and each time there is air in the system. I have check for leaks, checked the exhaust of excess moisture. What could be causing this?

Engine is a 2.0L HDi (90Bhp)

Engine temperature is good, stayign at 90C even under stress, and thermostat is operating correctly (opening radiator loop @ 90C).