can water pump be cause of MB 500 sel overheating during stop and go driving but stay normal at highway speeds? New thermostat, poly belt, fan clutch, radiator, and flushed. No head gasket problem.
Agree that you are probably not getting proper circulation.
If your cooling fan is not coming on, you will typically overheat in traffic, but not on the highway, unless the weather is really hot. So I would check the fan first. If the cooling system is clean and the radiator not full of bugs, it might be the water pump.
If you are really sure that all the above items ARE WORKING WELL, your radaitor may still be partially clogged or have badly corroded fins; that also will inhibit cooling capacity at higher cooling loads.
The fact that it stays normal at highway speeds would have me checking other things first. The first, as Docnick noted, would be fan operation. Heating up in traffic and cooling down while moving is a classic symptom of an inoperative fan. The other thing I’d make sure of is that the cooling system was properly purged of air during the recent work.
A bad pump would likely have you overheating on the highway too.
Maybe you should tell everyone the whole story - e.g. did you recently have all of this work performed and now this is happening and a shop is telling you its the water pump? And was all of this other work done to solve this very problem - or is the problem new?