Unstable Error

[Mud-Dauber]

I'm trying to perform surface math, by subtracting one surface from another, and contouring the resulting "." surface. The surfaces I'm creating have a lot of variable points, but only about 10' of relief across the site. I'm getting an "unstable" error when I attempt to contour the surface with a kriging algorithm and the gridding process aborts. If I select the standard one, the gridding process completes. I'm using kriging on the primary surfaces, so using standard on the difference surface isn't a problem. However, I would like to understand the source of the problem.

I'm using Autocad 2011 and the corresponding Quicksurf 2011. I have a 64 bit Windows 7 Enterprise OS, 64-bit Autocad and Quicksurf, and 8 GB of memory.