Hi All,
In case the brake boost vacuum hose isn't the issue, you might look at the fuel injector o-rings. I had a chevy V6 with only 113,000 miles on it that had similar symptoms (lean, o2 sensors). I had just replaced the head gaskets (overheating problem), but decided not to replace the fuel injector o-rings since the car was low mileage. Figured it was a vacuum leak, but it only has two vacuum hoses so it was easy to rule them out. After racking my brains, I figured what did I have to lose, so I replaced the fuel injector o-rings, and the problem was solved. Cars been running like a top for the last 4 years.
-Jerry