Another thng that may have been overlooked, is the radiatior cap itself. Since its off to the side on the resvoir, and it probably didn't get replaced when you got the new radiator, it warrants examination.
Also non MB rebuilt waterpumps are known for not having the close tolerances that MB requires, (although I would think that this would cause more of a problem at idle, unless at high speeds cavitation occurs).
keep your cool!
03/83 300D 184k
07/73 280 160k