Crystal Reports XI Export to PDF

Hi, I have an ASP.Net application that prints a reports using

ExportToHttpResponse(CrystalDecisions.Shared.ExportFormatType.PortableDocFormat, Response, False, "CIC Preview")

This is and has been working great up on till now and only seems to be Windows 7 IE 11 issue. It works fine first time round but if I wait for around a minute and try and preview again it only shows a dark grey internet explorer screen. I also that after the above call it triggers the page unload event.

This one has me well and truly stuck.

Thanks for any help,

Roger