I have a 1999 Chevrolet Monte Carlo 3.1 engine, 82,000 miles. I have had trouble with it losing coolant. It started out as a slow leak and I could not see where is was leaking. The temp gauge would fluctuate...it would stay on normal, climb to halfway mark and then go back down. I would have to refill the reservoir once a day. Thought it might be the water pump, replaced it. Then it started losing coolant faster, you could see the leak, it looked like it was coming from under the alternator area and was not pulling coolant from the reservoir and the temperature gauge would climb and go almost to the hot.

 

Sounds like you need to change your intake gaskets...

 

Intake gaskets are a definite possibility.