We have a 2001 S80 T6 with 142,000 miles. We haveused premium unleaded fuel 95% of the time, and always as the car has aged. It has experienced slight surging at highway speeds, 65-75, for several years. All four O2 sensors were replaced under extended warranty around 98,000 miles. I firmly believe this was due to inferior quality gasoline. I live in Alabama and that brand is JetPep. I have dabbled at analyzing this problem for several years, since it doesn't always do it. Running some fuel injector cleaner seems to help some and definitely restores full throttle power to the engine. My best guess as to a cure, high quality, name brand gasoline. I think this engine is VERY sensitive to it's diet. Just offered as information. Any comments welcome.