Stacking with X5 PhotoMerge

[aoc46]

I've been experimenting with using X5 PhotoMerge to do image stacking. This involves shooting
multiple images of a subject in a burst, then merging them. The random noise in any single image
is thus averaged out, so the final image IQ has much higher and larger prints will be more
satisfactory.

PhotoMerge makes doing this quite easy, but I find that when merging more than three images at a
time, artefacts show up that make sharp vertical edges appear jagged. (Horizontal edges do not
seem to be much affected.) I have tried several combinations with a set of 9 images, but always on
adding a fourth one, the artefacts appear; they do not seem to get any better or worse with 5 or
more.

The image attached shows (at 300x) :
A. a typical single image from the stack
B. the result of stacking 9 images all in one pass.
C. the same thing done by stacking the 9 images in three sets of three, then stacking the three
results.
Note especially the difference in the vertical window mullions between B and C. (Note also the noise improvement vs. A.)

From what I understand from PhotoShop users, they can do this sort of stacking with layers, but
they have to compute and set different opacities manually for each layer. If PhotoMerge could do
the job successfully in a single pass, PSP could have a significant advantage for those of us
wanting to use this technique.