could it be the water pump??
sounds like a head gasket to me, I'd get it checked immediately.
if it blows water through the tail pipe and white smoke it is your head gasket or a cracked block or a cracked head. if it does not have white smoke or milky engine oil than it is more than likley your heater core. if you determine that it is the heater core have your system back flushed before you replace the hearter core it could be pluged.
Overheating can be caused by many things such as : the fan, the fan motor ,the water pump, or a fuse. Overheating is a trail an error task. since you already changed the thermosatate the next thing you need to do is check the fan motor.
I would suggest you check your oil immediately, it sounds like a head gasket or some other leakage. I doubt it would be water pump. If your oil is discolored do not run the engine.
