Is this the old XBox or Xbox 360? I am assuming the old Xbox.
Unfortunately there are about 6 different revisions made of that Xbox motherboard. Some did not work fairly well on the earlier Xecuter chips.
But since i am also assuming that your old Xbox has always been running smoothly with it and now acting up, i am guessing this is either a bad solder or your harddisk gone haywire.
Check your solderings very carefully by pulling them a little.
Then check your harddisk using a bootable cdr with the Executer software on it.