Grid shader

Hi all,

I'm trying to convert the bricks shader so it will generate a grid. So far so good, no problems with generating the grid.
The thing is : I would like to give an offset value to the shader for positioning/shifting the grid. This works, but at certain values the grid disappears and I can't figure out which of the functions is returning the wrong value which causes this.
Any ideas?

    Shader for generating a grid