Before you go and replace a head gasket (do both if you do one), try checking your oil for water/antifreeze. It will be a milky-brown or greenish color. Foam will adhere to the underside of the oil fill cap.

Although the smoke from the tailpipe suggests head gasket, the odds are extremely greater that your lower intake gasket will fail first. This can cause the same fluctuations in temperature and coolant loss. It will also result in power loss as water would be filling your oil pan way past full.

I'm actually quite surprised that almighty Maniac didn't suggest you do something as simple and free as check the oil first.