Without an example, there's a river of variables running through my brain :) (give you some to think)
1.
Are they with fixed width? if so,
that's not good, because space runs
out and they are forced to go down...
2.
They have space to be correctly
presented, but they still move down?
Have you looked for other elements
that are without space to be properly
presented and go down, and so, forcing
the div(s) to go down as well?
3.
Do you have elements with no content,
and with the same color as your page
background, staying "invisible", and
as so, being messing with the div
position?
4.
It appends only on IE8? If so, IE is
very weird regarding floated and
positioned elements, but above all,
css margin property really gets on my
nerv sometimes...
The best way to solve does problems is to use Firebug on Firefox, or Developers tool-bar...
If you can post your code, or provide a link, I'm sure that's an easy subject, but just like what appends with me, the developer just can see it... :)
Hope it Helps U!