Hi all,
I have had a similar situation where the dialog was opening where it should on the page, but the user was redirected to the bottom of the page (in IE if I remember correctly). Basically, I forgot to include the appropriate css to match the jQuery UI JavaScript library. By doing this everything was ok. I imagine it's something like that, where there are styles set on elements on the jQuery css that are not set in your own css.
To debug the problem so I didn't have to include the whole jQuery UI css, I made two identical pages, one using the jQuery UI css and one not and just checked what was different in the css via Firebug on Firefox and added these styles to my css.
Hope it helps.
Mag