[jhv48]

First, make sure your timing is spot on. Retarded timing will raise temps.
Thermostat should be a 180 degree thermostat. Not the problem here.

Sounds like Air is still in the system. If the water temp climbs to 210 degrees and sits there, there's a pocket of air sitting around your sensor and the water is turning to steam. Do a search on getting rid of the air in your cooling system and follow it.