Weird problem on website?

Couple of things that could be going on.
header nav {} has declared position: absolute; but lacks any positioning information like top: 0; right: 0; (you effectively take it out of the normal flow, but don't position it at a set point)

It has float: right; assigned but is overridden by .grid_19 to float: left;

nav should probably not have class grid_19 as it does nothing for it really.

#Orange has unsupported attribute 'postion: relative;' which I assume is supposed to be 'position: relative;' which is probably why you sometimes get odd behavior on this id.

h3 has no semi colon on the text-shadow line.

.post has 'Padding-bottom:' which may or may not parse correctly to 'padding-bottom:'

Generally speaking, you're using some kind of grid system, but not the way it is intended.
Example: .grid_19 earlier is floated right instead of using .push_xx or something to move it over.