I know I may be a female and have very little mechanic skills compared to you guys, but I own a 05 G6 with 124,000. Just like you I have had the problem since 20,000 miles. I finally found the solution to the EXACT same problem. Flushing the Throttle body will stop the surge when it idles. I have no clue why this took so long to find or how it finally stopped the problem, but my MPG also went from 29-30 to about 19-20 and the fuel filter was clogged. Replacing this brought my MPG back to a resonable rate. HOPE THIS HELPS SOMEONE!!