Sounds very much like a BMW that we had a few weeks ago. I had read about filler caps causing the problem but carried on checking anyway, We put the Smoke machine into the engine breather and it puthered out of the filler cap, removed the cap, blanked it off and hey presto, Smooth running engine and emissions started settling down.
Are these a similar sealed breather system. ?? ( Thinking about it i suppose all modern systems are now sealed )
Also on the BMW it could be caused by Leaking Crank oil seals etc.
In any case it sounds as though the fault is being caused by some part of the engine breather system as it builds up to the correct vac if you blank it off.