The model may show rear polygons "bleeding" from behind the front ones.
In this case, the z-buffer lacks the precision to resolve the difference between near and far polygons. Reasons for this can be due to insufficient hardware, insufficient software drivers, or the model could be shifted very far from the origin. If your depth buffer size is less than 32 bits, and you are seeing z-fighting issues, then an upgrade to your graphics hardware will be necessary to fix the problem. In rare cases the problem can be due to the model being shifted too far from the origin (typically distances of millions of units are required to cause this), and shifting the model back to the origin may solve the problem.
Other z-buffering artifacts can be caused by surfaces that overlap in the same scene, which can cause a shimmering effect when the model is rotated. In this case, making sure that the part is displayed in surface mode in only one displayer in the scene will resolve the problem.