I was crashing a lot but only when playing eGo Counterstrike: Source servers. Turns out, it was "Macromedia Flash" on my MacPro. I trashed that and all's good once again.
Can you check your error logs (e.g. open Console in OSX and search for "HL2" then scan for the thread that crashed). If in Windows I'm sorry, but I can't help you...but there has to be something similar to the Console crash logs.