Not at all sure, but I *think* your car has a duovalve with two units similar to the monovalve on US cars. If so, could you be looking at an early failure of the rubber diaphragm that's intermittently allowing full coolant flow into the core on one side or the other?
My car behaved somewhat similarly until it finally failed completely, when a simple (and cheap!) monovalve core replacement fixed it.

__________________
Craig Bethune

'97 SL500, 40th anniversary edition

'04 Olds Bravada (SWMBO's)
'06 Lexus ES330
'89 560SL (sold)


SL--Anything else is just a Mercedes.
(Kudos to whoever said it first)