Hammer Editor Freezing (Since "MP" Update)

I’ve had this problem since the “Source Engine MP” update. I’ll open Hammer and everything will be fine until I go into the entity editor/ object properties screen. Sometimes it will immediately freeze when I open it. Sometimes it will freeze immediately after I close the window. Sometimes, it even waits about thirty seconds after I close it to crash. I can’t seem to find a solution to this problem, as it seems to be fixed for everyone else.

Here’s some info:

I’m mapping using a custom mod which has HL2, EP1, EP2, and a separate TF2 content folder mounted into it.

Along with the already stated problems, it will also rarely crash when I’m doing simple terrain editing.

Some of the textures (such as leaves on trees) do not have transparency.