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Does he have a UDP? Is it possible that the water pump is going out? If he has a UDP its possible that the water pump isnt turning fast enough. Have you tried to hold the throttle at cruising RPM while the truck is sitting still to see if it still rises?
Make sure the belt routing is correct. The water pump turns the opposite direction as the crank.
Jeff
If he has a stall convertor, hopefully he has an auxillary cooler for it.
After I read the issue but before I read any responses my diagnosis was an air pocket. I have no issues with this any longer now that I vacuum fill these. The LS is so easy to get air locked because of the low tstat mounting.
I'll start with belt routing and move straight to doing what red said. Brand new pump probably less then 500 miles and isn't weeping out the weep hole... ( LS motors have weep holes right? Lol)