Surface HideCells  
For speed reasons Surface series draws all its cells without sorting them in Z order. This fast method displays correctly unless the chart is in 3D mode with a large amount of perspective ( ie: Chart1.View3DOptions.Perspective := 100 )

The Surface HideCells property, when True, sorts all cells by Z order before drawing them.
This improves (eliminates) display artifacts due to perspective.
Note: Sorting time is proportional to the number of grid cells ( NumXValues by NumZValues ).
Series1.HideCells := True