The scale of the sphere compared to the vert spacing is much too close. You can get away with much more spacing between the verts which will help with the bumps. You can also try using 'friction pyramid approximation 1' by setting the dContactApprox1 flag with dContactSetMode() to help with any bumps.
The problem though is the flat TriMesh. When the sphere rolls over the verts, ODE detects these as rigid edged areas causing the bump effect. TriMeshes aren't expected to be totally flat in ODE.
There are two things which would be better suited, either make the vert spacing much larger (even creating a TriMesh from a simple quad would be better), or create the flat area using a flat box instead of a TriMesh. Another way depending on how your simulation is set up, would be to use the ground plane as the flat area.
The problem though is the flat TriMesh. When the sphere rolls over the verts, ODE detects these as rigid edged areas causing the bump effect. TriMeshes aren't expected to be totally flat in ODE.
There are two things which would be better suited, either make the vert spacing much larger (even creating a TriMesh from a simple quad would be better), or create the flat area using a flat box instead of a TriMesh. Another way depending on how your simulation is set up, would be to use the ground plane as the flat area.