One other tidbit of information.. It 'improves' the performance and reduces artifacts if I directly set the meshNum in the vertexshader to 251 as well?! So it's getting corrupted in the vertex shader as well. If I debug any of the vertices in a PIX frame capture, the meshNum is correctly 251 in all of them even without doing that override. It's like PIX doesn't know what's going on.
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The interesting thing is is that with this override, the corruption is much more infrequent - in fact, when i tried to fraps it and the framerate went down to 60 fps, I couldn't get it to happen - but it happens every 0.25 second or so at 1000 fps.
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Granted, this doesn't really matter for fixing the problem, because in the end I can't force a value, it needs to be the one it gets from the vertex format.
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I wish I could take more video - but anyway the video I posted is pretty similar to what is happening
[/font]This sort of thing makes you want to give up programming altogether.