It sounds like you're experience z-fighting, which means that the z buffer doesn't have enough fine-grained precision. The cause of this is setting the near and far planes too far apart. What values are you using? Try moving the near plane further from you (higher value) or moving the far plane closer (lower value). Moving the near plane will make a bigger difference, but changing either will help.

Of course, if you don't need the depth buffer and it makes your thing quicker then don't bother changing anything.

EDIT: Oh, and check the depth function mode as suggested by Paulius.