What blitz said. Check the memory first with memtest86 or something. With this kind of problem, I'd probably go through the following list:

• Check the memory
  
• Clear the CMOS data and go through the BIOS settings
  
• Check for a BIOS update from Shuttle
  
• Swap the IDE cable to your CDROM drive with one you know is good (I have had a number of Shuttles with iffy cables).
  
• Try another CDROM drive

There are a few more things to try, but the probability of fixing it is diminishing after the above list. Let us know how you get on.