Sounds to me you have non Planer geometry.

This can be caused by bad face extrusions or making a mistake on duplications. I would run Polygon->Cleanup [Options] ensure you have the non-manifold option checked and apply it. This will check the model and show you any issues.

Also you could check the normals to ensure they are all OK too. I had a lot of these issues when I first started with Maya and it was always things like having 2 faces occupying the same space etc...

Hope this helps.