I have the same problem, It freeze right after the player dies while a Doom NPC is present. I posted a comment on the Hammer Editor Discussion yesterday so hopefully someone can help. The only thing I know is that it might freeze because of the absence of a certain entity in the map or its a visleaf problem, but im not entirely sure

I think I found the solution. If your custom map's problem is related to the same cause as mine, then this should work. The problem for mine was a leak, I used custom textures that were not assigned a "$surfaceprop" variable in their VMT's which will cause your map to leak through the textures. If you have custom textures then add "$surfaceprop" "[material choice]" to each of your VMT's. If you dont have custom textures, check your map for any leaks using Map > Load Pointfile. I hope this helps you out

thanks