I have this problem again. I reported the same bug back in alpha 7 or 8, just as the game went paid, and it is back just as I paid and downloaded alpha 12 (dl Jan 18th).
I'm unable to play for longer than a minute or so before my framerate drops, game locks up, COMPUTER locks up and I am forced to hard-reset. The problem seems to happen immediately as I start selecting areas to be dug. Changing the process to use Windows 7 Compatibility mode gives me more time to play, but the process still crashes. I have retrieved some Event logs showing the crash, keeperrl begins hogging memory (3+gb so far) until it maxes out the system, then references an inaccessible ram location and crashes, and also causes my Service Control Manager to crash and drop all of it's managed services, essentially killing the machine.
[Windows successfully diagnosed a low virtual memory condition. The following programs consumed the most virtual memory: keeper.exe (4464) consumed 313057280 bytes, explorer.exe (3524) consumed 78172160 bytes, and svchost.exe (1256) consumed 51970048 bytes.]
I'm using an i5-4200U 4gb ram laptop with Windows 8.1.