[URGENT] Strange crash

[Damascus]

I'm alpha testing my latest maps, and for some reason, the game is crashing occassionally when I change maps or when I resume a save file. It doesn't happen all the time, only sometimes, and I'm not sure what's causing it. I really have no idea what could be causing the crash. The map isn't very big, though I have noticed a notable frame lag when I play through my most crash-y map.

If anyone feels like giving the whole thing a go and seeing if there's any issue they can find, here is the group of maps:

http://www.mediafire.com/?5u2nwcaj15qe6kl

Here are the main issues:
1) Map 3 (West Wing) is giving me lots of frame lag. I don't know what might cause this, as the map is pretty small and there shouldn't be a lot of polygons.
2) Using the level door leading from Map 3 to Map 4 (Basement) crashes the game about 50% of the time when Map 4 is loading.
3) Resuming a save file in Map 3 crashes the game about 50% of the time.

I've had one beta tester report the same issue as me, and another report no issues at all.