1996 Dodge Neon

I have a 96 dodge neon that seems to be overheating there is water in the radiator. No visable leaks. If it sets at idle it does not overheat for awhile only when I drive it. Then it only takes about 7 minutes before it over heats. It is a 4 cyl. And has 100000 miles. Head was replaced 3 years ago. Water pump has never been replaced. It was leaking but we put a sealer in it.I was just wondering if it could be bad and not pumping like it should when you take off radiator cap the water seems to move but not very fast even at running tem. Nothing is wrong with the thermostat