I've found most of the png fix scripts don't play well with dnn at all. I've seen many skins that have the scripts but the png's still don't render in IE 6 or lower anyway. I'd test the site in IE6 to see if the script even works. If not, then remove it and see if that resolves the issue. I've also noticed that some of the big time brand name sites are simply using png files with no attempt to fix them for IE 6 or lower. One of those is Comcast.com. You can't even use any input boxes unless you are using IE7 or higher. From the designer standpoint, what happens when we have IE 25, are we going to be implementing thousands and thousands of hacks for all the different versions, or do we design for modern browsers and recommend that our website visitors maintain their computers internet browsers to the latest versions because that really makes the most sense anyway, especially looking at security fixes etc. I'm on the fence with that issue. Some intranets literally can't be upgraded because of version specific requirements, but still, once you are several versions behind isn't it time to upgrade anyway and just deal with the upgrade process again for security reasons more than bug fixes? Of course, with IE, an upgrade only means new and improved bugs and new security leaks!
Anyway, you might consider just deactivating this png fix script to see if it is the culprit. You should at least rule it out, and move on to the next item to test.
I found that adding a lot of modules on a page can seriously slow down the page rendering and response times of menu mouseovers in IE8. So, I added the metatag to force IE7 compatibility mode in IE8 to the default.aspx page and that resolved any performance issues. I had gone back and forth with the core team, but no one could find a specific reason why IE8 was having problems with response time and slow rendering, so it was basically left that forcing the IE7 compatibility mode was the only solution for now until someone stumbles upon the actual cause for issues with IE8.