I have the Desire S. Bit long in the tooth, but it's not crashed ever. Neither has it ever received an over-the-air update (or any update, sigh) in the two years or so I've had it.
Personally I'd be looking at apps, even to the extent of malicious ones.
FS Corruption's also a possibility, but the Service Update makes me suspicious as I've never had one for the S. (C could be better supported, I don't know)
