Odd Banner Advert Behaviour - Please Help!

[donkspurs]

I've been using Sryrus' modification here.

Trying to position an advert in the main banner that will look good across all browsers and resolutions but I can only get it looking right in Internet Explorer. In all other browsers I get some odd behaviour.

The more I stretch the browser window out, the advert follows it correctly to the right side up to a certain point and then it suddenly jumps back over to the left and then continues moving right again. I can't work out why it does this, but it means for resolutions on 1280x800 the position of the advert doesn't look nice.

There seems to be a difference when logged in compared to when logged out. When I'm logged in, this strange behaviour with the advert isn't there and it correctly moves to the right side as I stretch the window out. It's only when I'm logged out that it causes trouble.

Here are two screenshots, from within Chrome. One is logged out, the other is logged in. If anyone can help with this it would be greatly appreciated - thanks!

Logged Out Screenshot

Logged In Screenshot