I’ve seen that problem with IE6, and added some code to fix it there, but I don’t have IE8 to test with and I’m not sure what the underlying problem is that would be causing the problem in that browser. Do you have any JavaScript debuggers installed in IE there? I’d be very interested to see if any errors were thrown.
This has been a problem for me with photos in a SLB but not for streamed video. I don’t get it, but if you click on a trigger for a SLB photo a second time in IE7 & 8 it will almost always open to the correct size in the lightbox. Unfortunately, visitors to your site may not know that.