Steam Community crahses this game.

[a wingless monkey]

I have determined that the primary cause of this game's crashes when using Direct3D on Steam is the presence of the Steam Community overlay (the thing Shift+Tab calls up).

If running the game from Steam, with or without Direct3D enabled, crashes the game upon loading a map, but starting the game directly from the directory or other non-steam method does not cause a crash, then disabling the Steam Community for the game in the Properties menu should fix it, and allow safe and functional use of D3D, as it did for me.

I've seen this problem mentioned here, but no-one suggested anything further than disabling D3D, which is detrimental to the game's presentation. So having figured out this much more complete resolution to my problem, I am putting this out there for anyone else who searches here for a solution.