[Barnsnake]

You may have air in the system. It is even likelier since you don't have a "burp hole" in the thermostat. You might want to loosen the temp sender in the intake manifold with the front of the car as high as you can get it (any steep slopes around?) This will release the air pocket in the engine.

Are you running a coolant recovery tank?

Do you have a thermstat bypass hose? If not, you need one.