My rendering seems great and fine so far, except when I try to render a texture to a polygon (typically 4 vertices as a triangle list).

The texture is a very simple colourful rectangle:
[attachment=15788:001.png]

The above image is saved inside a 1024x1024 texture, so the Power of 2 is not an issue.

I render to a quad with an elongated bottom, and the centre of the texture get's a bit biased and wonky towards the left hand side, shown in the following image:
[attachment=15789:result.jpg]

I need the texture to be rendered evenly throughout the polygon, I need the following photoshopped picture as a result:
[attachment=15790:desiredresult.jpg]

I looked at PIX of the frame and the triangles are being rendered with the correct UV and vertices values
[attachment=15791:pix.jpg]

So I don't really know why it is skewing slightly to the left down the centre. Is it because of the triangle split down the diagonal of the polygon? Do I need to enable a blending method or something?