It seems like memory corruption, the most annoying kind of bug. Might take some time to fix, I haven't been able to reproduce it on my computer.
This is a serious problem that may not have a fix. I have a GPU that I still have running on SHA256 because it starts throwing massive errors if I try to use the same intensity on scrypt coins.
I don't think this is a GPU error, but rather an error in the CPU code somewhere.