Well, I've spent the last week and a bit trying different kernels etc, and nothing seems to be working. I've tried those Via kernel patches, both on with ACPI enabled and disabled in the kernel, but all to no avail.

Long story short, I've hit a brick wall. I think I'm going to have to put it down to an unfortunately-timed hardware failure. The machine was moved around a bit when it was making the transition from Linux to NetBSD, so there could be something there.

Anyway, thanks to those that helped and offered suggestions. I'll run the machine on one CPU until I can work out something further.

Thanks,
Chris.