Air bubbles in the coolant is the key, you most likely have head gaskets starting to leak compression gases into the coolant system.
As has been already mentioned the head gaskets as well as the intake gaskets on the 3.4 GM V6s are problematic. Anytime you run an engine and you can see bubbling in the coolant reservoir, or if you remove the overflow hose in submerge it in a cup of water and see constant bubbling you can be pretty confident that you have a sealing problem in the combustion chamber.
Since gases rise to the top of the cooling system (the hottest portion of the system) the heat isn't efficiently carried away to the radiator, causing the temps to rise slowly until you're in the overheating mode.
If you diagnose that there is in fact gases getting into the coolant, you've found your problem.
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Brad Yost
1973 T/A (SOLD)
2005 GTO
1984 Grand Prix
100% Pontiacs in my driveway!!! What's in your driveway?
If you don't take some of the RACETRACK home with you, Ya got cheated 
  
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