Duratec 2.5 P0174 and CEL

Well, the Mystique made it to Winnipeg and back - but the light came on again. This time it had a low activity code on the number 2 bank -
so I replaced the O2 sensor. Still getting the odd P0174 - at 750 RPM and 0 MPH with IAT of 93+ degrees in closed loop (according to the stored "snapshot") Intake gaskets have been replaced, PCV system checked(and new valve installed), Bank #2 O2 sensor #1 replaced. Car runs perfectly - including idle. There are no signs of a vacuum leak.
Any good ideas?
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