Instead of "IFrame" in modal popup, i suggest to go for showModalDialog" feature.
so, on client click of any button/ link (as you were using target control for modal popup), add below code.
So, this will open the screen specified in url as modal dialog. the previous screen controls are not accessible.
you can keep one button in that new screen, and on client click of the add below code.
this will close the modal dialog.
like this you can avoid the use of IFrame and this is once of the options using which you can avoid it and also you can achieve modality.
hope it helps.