400E cooling

I have recently purchased a 92 400E with 160.000 miles. My latest problem is running hot while in traffic. Moving down the road it stays at normal temp but when I get stopped the temp raises slowly. If I put the car in neutral and rev the engine, it cools back down. What would be the most likley cause of this?? It appears that the water pump and thermostat have been changed at some time in the past. I have no service records for this car. Appreciate any help you can give. Thanks.....