How to fix?
(I know this is a little late, but better late than never I guess.) This happened to me as well, I dug up this forum post: http://forums.steampowered.com/forums/showthread.php?t=841911 TL;DR: Set hl2.exe to run in Compatibility Mode for Windows XP Service Pack 3 if you have Windows 7 like I do, and that should fix it.
Recent runs
Level: Anomalous Materials
Level: "We've Got Hostiles"
Level: Anomalous Materials
Level: Anomalous Materials
