273 overheating after new timing chain set and waterpump

Hi again guys. I had a problem last fall with overheating so I installed a new thermostat and it ran nice and cool again, under half on the temp gauge even in hot weather in heavy traffic.
A couple of weeks ago I installed a new timing chain set, new manifold gaskets and a new water pump. Now it runs nice and cool on the highway and about half on the temp gauge in town but when I'm in traffic it moves 3/4 way up the gauge and would keep going if I didn't pop it into neutral and raise the rpm. I noticed that the old pump had a plastic impeller and the new one has a steel one. Does anyone know if the plastic impellers flow more water than the steel ones.
Thanks again for all your help folks, I'm gaining much knowledge here. Wally