I prefer the fixed background (even though, like you said, it leads to a jerky scroll), but that background image is a bit harsh... it's a little too bright and distracts from the words on top of it... and it's especially annoying since it's always right in the middle of what you're reading :P

and you really do need to do something about those code boxes... make them just a few shades darker or lighter than the background color and you should be fine... something like the boxes on these here forums:
Code:
                5
         3             7
     2       4     6       8