I'm definitely not overly knowledgeable on this.
But I had similar issues last summer.
In addition the water cooler started leaking, it was end of season so I didn't do much with it, just loosened the cap a little and let it mostly sit over the next few months.
We just pulled the pump out, and found that the pump base was wearing out and probably wasn't running very efficiently.
The other thing to check is you're cooling stack? Is there a pile of mud and wasp nests hanging out in your radiator?
Anyway, hope it's not your pump... It's allot of labour to get to it.
G
GMC Sierra 2500hd 2004.5 now with ARP studs