Sounds like an air pocket. I've never had that issue on an LS, but my BMW did that after a water pump swap. It took several bleeds to get it fixed.

OOps I meant to close this it was the gauge

LOL. My bad. I read the first page and missed all the updates. Glad you found the issue.

I bought a GM crate LS2 years ago. While poking around it, I shined a flashlight up into the water pump heater hose fitting. I saw a bunch of red core sand plugging it up! I think that's why GM took it off the assembly line and sold it as a crate engine, they probably had a batch of them like that. Always take a good look at new parts.