From the sound of your description, I'm guessing you mean it's spitting out the tank. If so you most likely have a compression leak, either a head gasket or injector cup is leaking. The lack of heat until you hit the throttle, then rapid spike up and going back down is air trapped in the top of the cooling system preventing engine heat from making it to them to open the stats, but when you hit the throttle it forces coolant up thats hot, and they open then the temp stabilizes back out when the air purges out. It's classic signs of combustion leakage into the cooling system.