lost texels (orthographic texturemapping)

Hi Folks,

i try to display some small texturemapped squares on an orthographic viewing volume.
It works pretty well expect that i lose some texels (always a whole line) based on the destination X and Y koordinates.

Or in other words: It looks like opengl forgets to draw a vertical and/or horizontal line of the texture. (The lost lines randomly appear somewhere on the texture. (not just at the border))

Why do i lose the texels? and what can i do against it?

Please let me know and
thank’s in advance.