Thermostat?

pull the thermostat and test. you should test every thermostat before installation heres how to do it you will need a thermometer that is capable of reading the specific rating of your thermostat you can purchase these at wal-mart or any parts store at fairly reasonable price about $5 You can test a thermostat by putting it in a pan of water and heating it up. You use a thermometer to see at what temperature the thermostat closes and opens up, and then you match that to the thermostat's ratings.if it doesnt open or close at proper temp. replace it and test new one before installing. also i would not use any thermostat unless it was a stant superstat or mr. gasket....goodluck and post results.. also check thermostat when you remove it it can be in backwards the spring on thermostat should inside housing attached to the top of the motor. if the spring on thermostat is inside the waterneck housing thats removed when replacing its in backwards this happens alot and goodluck...

How did you burp the coolant if the stat wouldnt open?

In addition to Bill's very helpful post, are you sure the system is fairly full? If steam pockets travel to the top of the system when the stat is closed (the top is arguably the stat itself), the steam will banter the back of the stat. This alone can damage the stat beyond being reliable anymore. A balanced stat has a shrouding to help mitigate this abuse.

If the stat tests ok, make sure the system is full and that you have a jiggle valve/hole in the stat.

Good luck.