When you run a colder than spec t-stat, it will initially open and then never close. You will have constant flow through the radiator, this equates to the coolant not having enough time to transfer the heat out of the system via the radiator. Put a stock temp thermostat in, should be 195 degrees, but check out what the parts house says. And make sure to have a 50/50 mix of coolant in the system. And no, 200 degrees is fine.