Does your service information list this engine as having a MAF, a Manifold Absolute Pressure sensor (MAP) or both?
You state that the post cat O2 sensor has a "poor reading". Could you elaborate?
The way I like to test AFR sensors is to use an exhaust gas analyzer (4 or 5 gas) and compare AFR or Lambda readings to what the PCM and AFR sensor reports.
What are your fuel trims doing? Could you give us fuel trim readings at idle, raised rpm (no load 2500 rpm), cruise (under load driving) and while the symptoms are present?
And while we are at, can you provide us with the voltage output of the MAP sensor at idle and at elevated rpms?