Hard system locks on 3.12.5+; possibly nvidia related

I have two very different machines that are both running nvidia cards, which is one of the few commonalities. One machine runs the nvidia 304xx legacy branch and the other runs the current branch. They are both on up-to-date ArchLinux installs.

I’ve had to downgrade both to 3.12.4 or less to prevent hard system locks. These locks occur both when the system is in active use and also when the system is idle. A correlation to any specific activity or event has not yet been noted. The older machine experiences the issue much more often. The system is not responsive to sysrq keys and writes no information about the locking events to the log when a hard lock occurs.

Has anyone else been experiencing hard locks with kernels >= 3.12.5 and the nvidia blob driver?

See also https://bbs.archlinux.org/viewtopic.php?id=175070.

Older machine, eMachines acquired for a few hundred bucks, experiences locks more often:
Cheapo AMD Proc, about 5 years old now
Cheap discrete nvidia GPU has been added to the PCIe slot for dual-head output

Newer machine:
GTX 685