I'm going to guess you're using some version of Windows Server or Windows 7? There's a configuration out there that a few people (like yourself) have, that I can't figure out. Because it doesn't do this on every version of Server, Win7, etc... but its the only versions of the OS I've heard of it happening on.
I've actually spent quite a bit of time researching it to attempt to figure out what it is, but... no luck yet. If you have any suggestions on what you think might be causing it, I'll look into it. But I can't replicate the issue myself (no server/Win7 machines), so its much harder for me to fix.