99 Pontiac Grand Prix SE: Low Coolant light comes on seldomly but the vehicle is overheating. I can not find a leak in or out of the motor. I have no heat in the car and Im losing a little bit of power, I'd say 15%.

I've been told it is an Intake Manifold Gasket since they are plastic the cold weather causes them to crack, but are there any other ideas out there?