render to alpha texture sux with NVIDIA cards !?

dear all !

i have a rather simple algorithm that renders an object to a texture. now i want to use several of these textures to fake some kind of “motionblur”.
i do the following:

glClearColor(1.0f, 1.0f, 1.0f, 0.f); //ALPHA 0 !!
glCopyTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, 0, 0, g_Viewport, g_Viewport, 0);

later on i render this texture via:

glColor4f(1.f,1.f,1.f,alpha); //alphafade
glBindTexture(GL_TEXTURE_2D, myTextureGLINT);

so far so good - works totally nice on ATI cards - but not at all on NVIDIA (latest driver - no question) - any ideas on this ???
do i eventually have to use some “strange” NV_ extension to make this work ??
please help !!!

thnx & many greetings

Hmm… did you set alphaBits to 8 in PixelFormatDesc during context creation?