Take a few minutes and check for any loose or worn vacuum hoses. They do crack and break and don't last forever. I used to have a 1975 Pontiac 455 CID that would eat up vacuum lines. I had a terrible time with another vehicle that had a cracked vacuum line and I couldn't find it because the crack was on the underside of the hose, so I couldn't see it. The O2 sensors only last 60-70K in optimal working condition, your is way over due for replacement.