This was from the MotorCraft ODBII manual

The HO2S is also tested functionally. The response rate is evaluated by entering a special 1.5 Hz. square wave,
fuel control routine. This routine drives the air/fuel ratio around stoichiometry at a calibratable frequency and
magnitude, producing predictable oxygen sensor signal amplitude. A slow sensor will show reduced amplitude.
Oxygen sensor signal amplitude below a minimum threshold indicates a slow sensor malfunction. (P0133 Bank 1,
P0153 Bank 2). If the calibrated frequency was not obtained while running the test because of excessive purge
vapors, etc., the test will be run again until the correct frequency is obtained.

What is excessive purge vapors............what causes it. Wondering if this has any link to the trouble i've had??

 

Your truck has a purge canister (used to be called a carbon canister). It collects fuel vapors from the intake manifold, fuel tank, etc. while the truck sits. When you start the truck, the PCM will control a purge valve and attempt to suck the vapors out of the canister and into the intake manifold. It does so based on engine temp, load, etc. The vapors can by very rich in fuel for some time. This will affect the mixture that the O2s see.

Excessive purge vapors isn't a "problem" by itself, it just causes a problem for the diagnostic procedure. Once the canister has been "sucked on" for a while, the O2s should be seeing a proper mixture determined by the MAF signal alone.

Hope this helps.