Do you have an overflow with a return? Do you have steam vent return plumbed?
The thermostat most likely isn't the issue. Steam pockets are more likely. Had this issue on my LS3 480HP --- and I spent 3 days tearing into it.... the culprit was no overflow tank and every time it warmed up - and cooled down - it sucked in air which eventually built up and created a steam pocket. That AND the fan was running slow.
I'd check your voltage at the fans and make sure they're cranked up -- this would show up even if you had a 20% voltage drop - say the alternator is cranking out 14.2V and your fan is only seeing 12V...
There have also been some issues with the water pumps loosing their impellers -- they have broken off right at the shaft - so the pump is turning but there's no water circulation. You should be able to see water circulating in the radiator.
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