Engine chugs and loses power after thermostat opens.

What is your question?

engine runs great untill thermostat opens up. then begins to "chug" and loose power to the point it dies. BUT! if you shut the key off (while rolling in neutral) and wait a couple seconds and start the motor again it runs great for a minute or so and does it again. i believe its a vaccume problem of some sort. not sure where to start looking.