Mobile safari position:fixed z-index glitch when scrolling

z-index is not reliable with position:fixed, as shown in this fiddle: http://jsfiddle.net/mZMkE/2/ use translateZ transformation instead.

transform:translateZ(1px);

on your page elements.

EDIT:
In your code,
Add this css:

.bla, .projects, .contact  {
      -webkit-transform:translateZ(1px);
      -moz-transform:translateZ(1px);
      -o-transform:translateZ(1px);
      transform:translateZ(1px);
}

and then remove z-index refs from those elements and .intro.