[Rich]

Perhaps the pumps are worn out?

I've seen some pumps replaced by a mechanic lately, the pump body itself was all worn out because of sand and things that have been sucked up and run through the engine over time (such as spending a lot of time going to shallow water sand bars, or sucking up lake mud when the water is low).

So when you replace the raw water impeller, be sure to check the surfaces of the pump housing to see if there is excessive wear.

Also, if the pump bearings are wearing out, it could allow air to be sucked into the system and could cause a loss of cooling as water wasn't being efficiently sucked into the system.