I have seen water pumps that get all gunked up. Is there any chance you mixed DEX cool and another type of coolant at any time. I think if it was me, I would pick up some radiator flush and do a real good flush of the system. When over heating, it is for the most part, poor coolant flow. This can be caused by a obstruction, blocked radiator, collapsed hose, stuck closed thermostat (although I will admit, I've never seen this), or gummed up water pump. Flush the system and that should deal with most of the causes.

No. There has never been any other type coolent in the same garage. But I think you are right , there is a flow problem. I will pull the pump off while it is empty.

I saw one one time where the impeller was not tight on the shaft, but he wouldn't get more than a mile or two before he was completely overheated. We went through all the usual suspects and finally decided to change the water pump even though there were no obvious defects. When we got it off and started looking at it we noticed you could hold the shaft with one hand and twist the impelled with the other. After replacing it , no more problem.

Fred