I gave up on ATi recently... The price / performance just isn't worth the grief of wondering if your issues are because of their hardware / drivers.
Sounds a bit like a corrupted memory module on the card itself.
Everytime the GPU tries to store or retrieve info from that one particular module then it either fails or comes back corrupted causing the stutter.
If it suddenly started happening, was working fine before, and didn't happen just after you installed/updated something, then may well be a hardware fault.
No driver upgrade will fix that. May well need a new card, though can take the card out and take it to either a spare PC or a local PC repair shop and have them test it for you. If it stutters with them, then it's bust.