From everything I've read/found about the problem, including email Sonnet, the consensus seems to be a bad L2 cache chip, or at least an easily annoyed one. I've tried 7.6.1, 8.5, 8.6, 9.0.4, and 9.1 with the newest version of the extension, loading first or last, and nothing works. I've even launched with nothing BUT the Sonnet extension on to check for conflicts but it still crashes. Sometimes it gets to the desktop, but within 45 seconds it's frozen again. For what I'm doing with it (writing and the occasional old game), it hasn't bothered me too much with the cache off, but I know I'm missing a lot of potential performance.

I hadn't thought about other manufacturers -- I'll have to look that up, and maybe try the original driver that came with the card.