Yes, well, Dark Valley -and also Army Warehouses- levels have
vanilla level design problems which may exhibit themselves in varying degrees. Choppiness can be experiences at different proportions, or in unnoticeable amounts. All depends on the hardware, and also the software, combinations. Significantly, X-ray has a memory management problem, and a lot of high quality RAM may make a visible difference.
For example, when I upgraded my OS to
Win 7 64 bit, the general stutter seen throughout the game (a vanilla STALKER symptom) had completely vanished! So 64 bit environment is somewhat a natural habitat for STALKER; interesting,but true. So, software is important.
Also, as we all know, a Graphics Card -no matter how powerful it is- cannot pull all the ropes by itself. It needs a adequate Power Source, a powerful enough CPU and a lot of RAM (very essential in gaming)
I, in average, have no less than 40 frames (even in the thick of the action) in Dark Valley and in Army warehouses levels. But I run
Intel Quad @ 3.15 GHz, EVGA 260GTX SSC (super OC'd), 4 Gig RAM 1066MHz air-cooled Corsair Dominator, ASUS PQ5 Mobo @ 1680 MHz, Corsair 800W PSU, and temps never go beyond 49-50C marks. Plus Win 7 64 bit shows that it's very helpful toward STALKER. --
On stretching NPC images: well, it's a physics engine matter, Mod does not do anything on this angle. However, as the mod (which is actually a multi-mod mega add-on, which also incorporates a good part of Inferno, too, among other mods) is very huge and makes the vanilla STALKER extremely demanding beyond all what had been intended by the developers, some glitches/coughs can be expected, depending on the overall power of the PC that's running it.
I see such incidents
rather unexpectedly in Fallout 3 as well,

but they never bothered me; they don't change the gameplay in any bit at all.
All in all, as far as I can gather, I may suggest that you may be having
1) a PSU inefficiency to feed the Card and other components,
2) a possible memory leak.
Beside offering a well expected and much used recommendation of upgrading those components, meanwhile, you may try check tweakguide site for a complete list of STALKER tweaking; it's known to help with many problems at certain degrees.
Hope you can continue to enjoy the events in Cordon; and, also in the rest of the game.
