My guess would be a combination of Shadow Min Bias too low, plus the mesh construction itself.

When the Min Bias is too low, polys can end up casting individual tiny sharp shadows. Is it set to the default 0.8 on the spots or is it lower? Also, depending on the version of Poser, I've had unexpected results in later (P9 & up) versions when using spots with shadows set to anything other than 1 (though I don't think that's the issue here) - are the shadows on the other two spots on or off?

When using dynamics I've found that a regular grid of quads will give a sawtooth-looking result as soon as a fold goes against the poly flow. What I mean is that the poly flow on the mesh here is all in straight lines. Bending only happens at the edges. When a fold runs against that it it looks like stairs or a sawtooth pattern. If the fold is large and subtle or the mesh is very high-res it won't be very apparent, but when either the mesh is low-res or the fold is small and sharp, it becomes very obvious.

A close-up render of the problem area - plus wireframe too - would be interesting to see if possible.

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