It is sometimes hard to get the results you need in a situation like this but frankly since this problem started right after installation of a new head gasket then guess what? More than likely it is the new head gasket unless this was a problem that had developed before and this is why you did the head gasket. In which case the head gasket was not at fault and you are now chasing the problem still.

Anyway, you may or may not get satisfaction from this mechanic but I agree wth ferdman, have the 2nd or even go a third reputable mechanic and if he finds the same issue have him call the first guy and explain his diagnosis. If he balks, go to another perhaps until he gets the picture...

just my .02

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'99 S420 - Mommies
'72 280SE 4.5 - looking to breathe life into it
'84 300SD Grey - Sold
'85 300SD Silver - Sold
'78 Ski Nautique