Avree with @
Kodename47 its often very difficult to diagnose ecutek tunes as ghe tuner may be using speed density or a series of custom maps to control fueling in ways we dont understand or know about without access to the tune istelf.
Its certianly possible the o2 sensor is bad, its a little strange it only intermittent, usually thats a connection or earthing issue, but again possible it bad 02. Since they are fairly expensive best to cgeck out other possiblities first