A check engine light mystery

I have a 1997 - 850 Volvo sedan. About a year ago my check engine light came on. Took it to Auto Zone and the code pointed to a faulty oxygen/air sensor. Had it replaced and drove the car for about 300 miles at which time the check engine light came on again. Went back to the mechanic, part was under warranty - he changed it again. Drove it another 400 miles (couple of month) - CHECK ENGINE LIGHT ON - again. Took it to another mechanic (spec. for Volvo) and he said one of the circuit wires in the back was loose - he fixed it. After each of these event the check engine light is off> until another couple hundred miles - light is back on. However this time when he plugged in the diagnostic little handheld gadgets the light turned off for about 60 miles during which time I ran off to emission inspection. Lo and behold, car passed emission. Drove out of the station and within 50 miles - of course Check engine light is back on. What on earth is this.... what do I do now?? (BESIDES BEING TOTALLY GREATFUL for a passed emission insp) Thanks for all input!