Well, I get the cards by name and they don't change, but I can't use both USB and the normal AC97 at the same time.
VIA8237 (disabled in bios but still showing in mixer)
This is plug and play - the operating system sees the thing even if it's disabled.
It could be that the USB card (most likely - or the other card) is sometimes initiated before and sometimes after the other sound card. Unplug the USB card and pug it in when the boot is fully completed and see if it still changes - I would not think so...