Couple of thoughts. If the temp shot up that quickly it could be just hot water that was in the block moving to the area where your sender is and giving you a temporary reading. The other thing is that 210 isn't all that high, did it want to go higher? Could also be an air lock in the block.

Do you have a shroud on the mechanical fan? I'm really a fan (no pun intended) of electric fans with a good shroud, especially for the low speed cooling ability.

Start it up and see what you get this time.

Don