When I scroll down the browser page in Firefox and Internet Explorer and then I start to drag igDialog, it jumps down and it appears with offset from the mouse pointer and stays this offset during it's dragged.
This issue is caused by the fact that igDialog uses jQuery UI draggable framework and there is a bug in its version 1.10.3. If you scroll down the browser page and then start to drag the dialog, it jumps down and the offset is the same pixels as the scrolled offset. It's recommended if it's not necessary to use version 1.10.3, to use older or newer version of the framework. The issue is not presented in 1.10.2, but if version 1.11.0 is available and you can use it, just do it, because the bug is fixed there. If this is not proper solution for your case you can also follow the workarounds proposed in the jQuery UI ticket.
Out of curiosity have you tired the work around suggested in jQuery Ui ticket 9315?
I am using jquery ui 1.9.2 and experiencing the same issue. Tried work around and it did not help.
Thanks