GRUB Error 17

Every time I update the kernel image, GRUB's menu.lst file gets altered so it doesn't which partition to mount first. I have my linux root in the third partition on my second disk, so the GRUB's root should be (hd1,2); when I change the kernel, it always gets changed to (hd1,3), resulting in a GRUB Error 17, cannot mount partition. I thought I'd trashed the system at first, but it's relatively easily corrected by using GRUB's editor to change the line root (hd1,3) to root (hd1,2), and the pressing b to boot. I then need to edit menu.lst to correct it more permanently. It's a bit of a weird error though.