P0140 code

98 lumina, 3800 series 2
P0140 code keep's popping up, Only after a long idle.
If code is cleared and you can drive a full tank of gas out of it with out the CEL coming on.
O2 sen circuit no activity detected. Bank 1 sen 2. This is the post cat O2 sen.
Has a issue with a broken MAF sensor, Tip was broken and bent sensor wires, Swapped out the MAF sen and code comes back.
I monitor the O2 sensor with my Snap-on Scanner and it's got activity,
Other than removing the O2 sensor and wiring up to the connector at the motor, and checking it out, I'm at a loss.
Motor run's fine with the CEL on, Suspect it's just intermittent and another word's not active all the time.
Although the car has a aftermarket Midas Cat on it and i just replaced the LIM gasket's, Plenum and valve stem seal's.
It's also possible the Rear O2 sensor is oil fouled from the bad valve stem seals.