Thank you all for the prompt replies.
I am beginning to suspect the MAS or an intake leak too, because we did remove the cross-over pipe (from the air cleaner to the intake manifold) to get at the plugs the other day.
Where did your leak occur Suginami? The two ports at the bottom of the cross-over piece seems deformed on mine. I re-inserted snugly it and now it seems ok but time will tell. BTW How do I test a MAS without a diagnostic computer? I am quite far from the dealer...