That sure sounds like you have an air bubble in there. I don't know if it was left in there from the thermostat change, or is being "injected" by something else (hope not head gaskets). But I would definitely try to bleed any air out of the system and make sure the hose between overflow tank and the radiator cap is not clogged. While you are at it, if you did NOT replace the radiator cap recently (in the last year or so), I would do that too.
To bleed the system, use the two bleed screws - there is one over the water pump (on the right side of the engine) and one near the thermostat housing. You can see some pictures of the locations
here.
Good Luck!