[hrydgard]

Maybe a cache coherency issue, where data written by the CPU is not visible to the GPU, without some kind of explicit cache line invalidation, maybe on both sides? Caveat, I don't know much about the memory system on the XBox but this is a typical issue with homebrew code on consoles.

[badsectoracula]

Yeah i thought about it and you can sync the GPU and CPU (basically wait until the GPU stops doing anything) and even bruteforcing a sync before and after every register change it didn't solve things :-/.

It might still be something i'm doing wrong GPU-wise, but these flickers happen with static geometry (the memory for which is allocated at init) -seemingly- because of "damaged" transformation matrices (modelview and projection) passed to the vertex shader. The matrices are inlined in the push buffer (i.e. become part of it) that contains the commands to setup the render state before the command that draws primitives using them, so basically it is impossible for the GPU to "miss" them :-P.