Is it a Honda thermostat or an aftermarket one?
(someone in a prior topic said the OEM thermostat is very well made; as opposed to an inexpensive aftermarket thermostat that saves you $10 to $20 and gives you headaches)

Also, what temperature thermostat is currently in your car? a low temp one will cause overcooling.

If the coolant returning from the radiator is cooling the engine too much, the thermostat should closeup (keeping the engine at proper operating temp). (maybe, the one you have isn't doing that.)