theres really no such thing as a bad water pump (the mechanical parts of the water pump are nearly bullet proof [gear, shaft, impeller, bearings] on a super rare occasion the bearing may go bad this would mess up the seal and you would have a coolant leak right away....so its safe to say that as long as the pump is not leaking coolant, than its fine. if its not the thermostat then a big bug crawled into your cap opening and is blocking a passageway....dude all signs point to the thermostat...and that air bubble thing is the funniest ive ever heard.