I have freaked my game out many times creating crazy items but I think I have the hang of it now.
In my current game I created two items. First I modified the Staff of the Magi (added a couple extra lightning/fireballs, upped the spell trap to 30 levels, etc). Next I created a ring that doubles 1st through 5th level spells and adds 4 extra spells from 6th through 9th.
Is it just me or is the game more stable if you created items and then start a new game as opposed to creating an item once you are already into the game (like in chapter 3 or something)?