it has to do with the render size of your flash, set the pixel size of the flash texture in the GDITexture with the inspector. or you can try to use Flash (EX9) from the addonpack, it outputs a texture directly and supports alpha.
@diez: it’s strange. i don’t understand the problem, because i’ve tested you model and it seems to be perfectly working with the flash texture (same as a simple texture at least).
and for interact with flash you have two methods. The simplest one (often is sufficient) is to use the input pins for passing it things like the mouse coordinates or the mouse clicks, ot keycodes. And the second one, for elaborated thing, to use osc send in vvvv and flosc for listening the osc and converting it in something usable in flash.
anyway, for your problem, can you post a screenshot of the problem?
ps. i think you already resolved, anyway for use the flash texture in the shader you must attach the layer output of flash in a renderer(EX9), then attach to a dxtexture, and that’s it. i attached an example