2D in 3D Texture Woe using C#

[Balaam]

I've come across a texture problem in my 2D tile game in 3D.

I know about the texel pixel issues when trying to create a 2D game
using 3D quads described here:
http://msdn.microsoft.com/library/default.asp?url=/library/en-us/directx9_c/directly_mapping_texels_to_pixels.asp

Basically I can render my tiles perfectly but when I move the camera,
(by transforming the view matrix) sometimes a border line of my tiles
textures will stretch to two pixels or vanish completely.

I've taken the code out and put it into a very simple program to
demonstrate my problem easily.

Heres the source code: http://ravensmyst.com/balaam/Problem.zip

Someone suggested I clamp the textures - I did this and it removed pink
lines I was getting but the lines expanding and disspearing remained.

Here's a screen shot of what it looks like at the start:

http://ravensmyst.com/balaam/happy.png

And after moving the camera down a bit:

http://ravensmyst.com/balaam/mope.png

My wild guess is that's it something to do with the random manner in
which I've set the camera up. Can some one set me straight, please!