Overheating
 

I would do a compression check. I suspect even though you replaced the head gasket, the gasket might not have been the problem. I don't know if you had the head checked, but it could be cracked or warped. Also not all head gaskets are the same. I an engine overheat after a valve job. Turns out the head gasket was missing one coolant hole in the gasket.

Another possibility, if the steam has cleared up & the engine is operating ok otherwise is that you have air in the cooling system. I'm not sure on your car, but the radiator where you pour in the coolant is usually not the high point in the cooling system. You may need to remove a heater hose to bleed the system if the fitting is higher than the
radiator cap. I hope it something simple like this than you having to remove the head again.

Thanks for the info Bulldozer. I'm still clinging to the hope that the head gasket was the problem in the first place, it was definitely leaking coolant on the ground when we bought it and it's not now. The steam is starting to clear up now, all I've had a chance to do is let it idle for about twenty minutes and then drive it around the parking lot about ten minutes, can't get it out on the road until we get it insured and registered. I was thinking about air in the system too, I had that happen to my 99 Cutlass when I did the intake gasket but it worked itself out. I think I recall the Haynes manual saying the 4 cylinder Sables have a bleeder valve but not the V6, and thus I would have to remove a heater hose like you say. I will try that tomorrow!

Those are all good points, Bulldozer.
 

A couple more things that could cause this is a bad radiator cap (not holding pressure); or a bad temperature sender unit.
When the engine is running, does the coolant level in the overflow reservior drop? This would be a sign that the cooling system is sealed and operating normally.
Have you noticed any water deposits or residue near the weep hole of the water pump? Also, do you hear a sqeeking noise coming from the water pump? These would be indications of a bad water pump. It sounds like you're on the right track... good luck.:cool:

No, it either works or it doesn't. Have you had the battery and alternator checked. When you put a load on the engine, more voltage is required. A low voltage output from the battery will kill the engine.