Overheating problem

I know that this an A body forum, but I need advice about my daily driver, which is a '93 Chrylser Dynasty (it's a Canadian version of the Dodge, but basically the same car). The problem is that it overheats at low speeds, like under 30mph. I put in a new summer thermostat, but I still have the same problem. Do I need to replace the water pump? Thanks.