I got my '98 SSEi 2 years ago with 106 K miles on it. Previous owner, who I knew, kept all receipts for maintenence he had done on the car. I noticed he never changed the belts, so when asked, said he hadn't. Changed belts at 114 K miles. Last week when entering the Interstate, noticed lack of power and noticed the boost gauge never went up to zero. Lokked under the hood and idler pulley had disintegrated. Car ran o.k. so took it to the Pontiac dealer where I have purchased many cars and set up appointment. Replaced the supercharger belt and idler pulley and car runs great. Car only has 122 k miles now. My question is, after 8000 miles was the pulley bad when the belts were replace, or shoddy mechanic who changed them?

Neither. The pulleys just go from time to time. It had nothing to do with the mechanic or belts.