FIRST I CHANGED OUT THE THERMOSTAT, THE PROBLEM DIDN'T STOP. NEXT I HAD THE HEADS LOOKED AT AND HAD TO HAVE THE VALVE COVER GASKETS REPLACED. PROBLEM STILL NOT SOLVED. NEXT WE ARE GOING TO CHANGE OUT THE WATER PUMP. I WILL LET YOU KNOW IF THIS SOLVES THE PROBLEM OR NOT.
When this happened to my 2002 . I did everything possible untill It became obvious the the head gasket was the problem. It's fine now, I can't say the same for my knuckels and my back. Parts were about 4 hundred. Including having the heads resurfaced It took a few solid days to get it done. Buick engineers should be assamed of themselves.
You better get this looked at by a shop. Don't drive this if it's overheating, get it towed, you can do more damage if it overheats! Here are some in your area: http://repairpal.com/directory?address=02302&car_brand_names=Buick
I am having the same issue. Additionally, the 2nd engine fan is not coming on but will come on under direct power. Have checked fuses and relays and all seem fine. Have not changed water pump but I really believe mine is the fan just dont know why it wont come on