Update:

Removed gas cap when symptom present--no change.
Checked for unrestricted flow through cannister--OK

Swapped purge valves with known good vehicle. Caused idle surges and unstable idle in good vehicle, but symptom still present in problem vehicle.

Swapped valves back to original cars.

Gave a few squirts of WD 40 to cannister side of valve during run. (recommended in other posts I found on this subject) No change.

Is it possible that the valve is/was indeed malfunctioning but the adaptation has done it's best to correct? Wouldn't this caused an "adaption at limit" code to be stored?

I'll post if I come up with the answer.

J. M. van Swaay