There is a technical service bulletin related to this problem, I am sending you. If there is air in the system you will need to bleed the air from the system. I would recommend to change the thermostat, do the service bulletin procedure and install a new cap. Then bleed the system of any air. There is an air bleed made of brass usually sits on or near the upper radiator hose inlet on the intake.
Bulletin No.: 00-06-02-001
File In Section: 06 Engine/Propulsion System
Date: January, 2000
Subject: Engine Running Hot, Overheating and/or Loss of Coolant (Polish radiator Filler Neck and Replace radiator cap. Models:1999-2000 Passenger Cars and Trucks with Composite radiator End Tank
Some customers may comment on one or more of the following conditions:
The radiator filler neck may have an imperfection in the sealing surface.
Important: DO NOT REPLACE THE RADIATOR.
Using a piece of 400 grit wet/dry sandpaper backed with a flat piece of wood, polish the filler neck sealing surface using a circular motion.
Replace the radiator pressure cap with a cap of the same part number as shown in the GM Service Parts Catalog.
Is there a procedure for bleeding the air out of the system after it has been filled? Do you open the brass air bleed screw (I've seen it) as you're filling the system?
Fill the radiator up and let it run until the thermostat opens and add any more coolant to top it off. Once you know the radiator is full install the cap and open the bleed valve and run the engine until you have a good stream off coolant coming out and then close the bleed off.
OK - Thanks Jim! I'll do these things and hopfully this will solve the problem.
Don, (Pickering, Ontario, Canada)