Option to generating isosurface?

Hi, I have three 56x56 matrices X1, X2, X3 and a 56x56x56 'data' matrix (containing values between 0 and 1) attached to each of the three matrices' combinations. I used an iso-surface command to grab all the components in the 56x56x56 matrix that are below 0.5 and color them red. In the following manner:

 data(data<=0.5)=0.500000;

 fp=patch(isosurface(x1, x2, x3, data, 0.500000));

 isonormals(x1, x2, x3, Error,fp);

 set(fp, 'FaceColor', 'red', 'EdgeColor', 'none');

However a lot of the x1 and x3 values equal 1 and this causes issues when interpolating and creating the grid. I obtain the following errors:

 Error using griddedInterpolant
  The grid was created from grid vectors that were not strictly monotonic 
  increasing.

 Error in interp3 (line 138)
       F = griddedInterpolant(X, Y, Z, V, method);

 Error in isonormals (line 76)
        n(:,1)=interp3(x, y, z, nx, verts(:,1), verts(:,2), verts(:,3));

So I'm looking for a better approach at illustrating this 4D problem. Any comments or suggestions are much appreciated.

Thanks!