"You've got some blockage
"You've got some blockage going on. The t-stat probably isn't the problem, if it was stuck shut, the engine temperature would be hot and the risers would get more raw water cooling flow, therefore would cooler.
If it was stuck open, the engine would run cooler but there'd be no reason for the risers to get real hot, warmer than usual, maybe.
Once the risers start to rust, chunks flake off and start blocking the water passages. From that point, the water flow tends to 'channel' through the path of least resistance, the other parts get less flow and get even hotter, and rust even faster... sounds like it's time for new risers and manifolds."