The server is crashing when people enter the Nether, there are two many people on, or when using minecart systems in the Nether. This has been happening for all snapshots after the one that added slime blocks, on a server that previously supported several players being on at once. The server was crashing from even just entering Nether Portals before, and seems to crash within several minutes regardless of player actions. I upgraded to 64 bit Java and allocated 1.5G of RAM (instead of 1G) and things improved slightly, but the crashes are still happening.

If you would like the server log I can post it, but I don't want to hand out any IP information if it can be avoided.