Lots of good advice there, but the single most common cause of this behaviour is the fuel pump relay.

A friend of mine recently had an identical problem that was eventually traced to the fuel pump - it's poor performance was the cause of a weak mixture and bad O2 reading.

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Mick J
'08 Chrysler 300CRD (MB OM 642 engine)
'95 E220 estate
'89 230TE (R.I.P.)