Well thanks for the various suggestions.
non conventional viewing size...sampling... sharpening... and the winner (or in this case loser) is.... sharpening
First let me say that I made a mistake in my previous post. My sharpening settings in the offending version were in fact Sharpness 51, Radius 2.5 and Detail 25, not the oversharpend (140, 1,99) that I originally indicated. I must have been looking at something else.
Anyways, I reset the (51,2.5,25) to the ACR defaults (25,1,25) and reconverted to jpeg (at the maximum quality,which is what I used for the offending version) keeping all other modifications as is. The graininess that I saw in the original jpeg at factors greater than 25% effectively disappeared.
Using "normal" (33%, 50%) settings or "odd" (25.1, 26%) settings for the zoom factor did not seem to make any difference whatsoever on either the original image (normal settings did not improve the graininess) or the revised image (odd setting did not degrade the graininess).
Note that when I look at the original Raw image in PS before conversion to jpeg, I do not see any issues. So the culprit seems to be sharpening+subsequent conversion to jpeg. Effectively the less the sharpening, the clearer it is.
Nevertheless i still find the degradation from 25 to 26% in the original version a bit odd...
Thoughts anyone?