I have heard that non MB rebuilt waterpumps do not have the close tolerances that MB rebuilt pumps have. Could your pump be non MB rebuilt? (that only when youre going 90 the engine revs fast enough to pump the water fast enough?) Once the engine is warm, do the hoses appear to be pressurized at least a little? And, with the engine off, are there any cool spots on the radiator when it obviously shoud be hot?
Check these things and post to your hearts content.