It's probably all the various widths you have specified conflicting with each other. For example, you have #mainWrapper set to 993px, but #contentMainWrapper, which is inside #mainWrapper, is set to 1011px. I'd start there.

Also, you have the site set to force IE7 emulation, so we don't know if it'll work in IE8 or not. I'd remove that.

<meta http-equiv="X-UA-Compatible" content="IE=EmulateIE7" />