500sel overheats

[mike500sel]

I recently purchased a really nice but overheating 500sel. I've swapped out the thermostat, also ran it without the thermostat, put in a new waterpump, visco fan is working, radiator has good flow, plenty of coolant, rad hoses are in good shape. Car still overheats. When the car is cold, I can start it and it takes quite a bit of time to get to 110*C, if I start it and drive, it will overheat faster, suggesting a coolant flow issue. Top hose can get pretty warm, bottom hose stays cool. Radiator doesn't get particulary hot either. The heater puts out plenty of heat and doesn't seem to have any effect on the temp gauge. I've heard something about an auxillary water pump, but I can't seem to find a paper manual for this car so I can research further. Any help would be appreciated