The water pumps aren't usually the problem on that model. At least not a flow problem. I have seen POS aftermarket pumps with plastic impellors. Some manufacturers BMW, Jag, and Porsche actually put these problems on cars, but MB hasn't sank that low yet. Plastic impellors have been known to slip at certain engine speeds.

But, it really sounds like a radiator problem.

__________________
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician