Chris,
Alright, tested a bit on XP. With IE7, the page works just fine. With IE8, I'm intermittently seeing the issue you mention. It happens the majority of the time, but not always. After the page is finished loading (even if not correct), if I do Page->View Source, the HTML just cuts off. It seems like your server isn't sending the whole contents of the page.
While this may be caused because you've enabled the JFBCSystem plugin, I actually believe that's just what's exposing a bigger issue. What I think is happening is that your server may be running out of memory rendering the page. There are some ways to easily test this theory. Why it's only happening in IE8 on XP is unknown to me.. do you have anything that changes your sites behavior based on user, browser, or OS? Many templates and/or 'optimization' scripts may do this. Either way, some ways to test if this is the issue. Try reloading the page after each step, and see if there's an improvement:
* Disable Error Reporting - Global Configuration->Server-> Error Reporting = None. This reduces overhead, slightly, and may correct for a minor issue in the JFBCSystem plugin.
* Enable Caching. This can reduce overall pageloading time and resources required. Global Configuration -> System -> Cache = Yes
* Disable (temporarily) your SEF component. Can't tell if you're using built-in Joomla or something else, but try disabling it (quickly) to see if the issue goes away
Again, I've tried repeatedly, and do not see this in Win7. So, it looks to be XP/IE8 specific. Not good, but could be worse.
If this doesn't help, or at least narrow some things down, let us know. I don't know if you have a development site or not, but if we can get super admin access to the that (or the live site), we may be able to debug this pretty quickly with just a little testing. If possible, please Private Message us account details.
We'll help you get to the bottom of this, and sorry for the troubles. Definitely a strange issue. If you know anything 'unique' about your configuration, we'd love to hear it.