If you have any 2D as well as 3D then make sure the following render states are set before displaying the 3D object.
GraphicsDevice.RenderState.DepthBufferEnable = true;
GraphicsDevice.RenderState.AlphaBlendEnable = false;
GraphicsDevice.RenderState.AlphaTestEnable = false;
GraphicsDevice.SamplerStates[0].AddressU = TextureAddressMode.Wrap;
GraphicsDevice.SamplerStates[0].AddressV = TextureAddressMode.Wrap;
If there isn't any 2D in your demo then check your near and far planes
Matrix.CreatePerspectiveFieldOfView(MathHelper.PiOver4, aspectRatio,
1.0f, 10000.0f, out projection);
If your near plane is below 1.0 then change it to 1.0.
The last thing is to make sure your culling is set correctly (CullCounterClockwiseFace).
If your problem isn't due to any of those let me know.
Thanks,
Chad