Sounds like either a buggy video card driver (in which case, reinstall it or upgrade to a new version of the driver if available), or else your video card might be dying sadly - I had a similar problem back in my early days in FFXI and it preceded total hardware failure of my video card (the actual GPU chip overheated and physically melted) - in the weeks leading up to that, I used to get random flickering of textures and weird graphic artifacts which only worsened over time. Replacing my video card fixed the problem (and I've never had a card die on me like that one did).