Grand prix heat off and on also, drinking coolant?

02 grand prix 250k miles. Really weird situation here, what it does is the
gauge will act normally and produce heat normally. All of a sudden it flies
up and down slowly almost overheats but doesnt. Then the gauge will
come back down all the while me having absolutely no heat. What ive done
before is add coolant (dexcool) And it will stop, it will produce heat, and act
normally for about two weeks. Now ive noticed no physical spiils on the
ground and the oil isnt milky. It needs changed but isnt milky. any ideas???
im at a loss here...