Similar problems here.

Using: 331.38, 331.20 or 304.19 drivers (32-bit libraries also installed), Fedora 20 x64, GeForce GT 240, 3.12.8-300.fc20.x86_64 kernel, GNOME Shell 3.10.3, X.Org X Server 1.14.4, GDM 3.10.0.1.

The screen freezes, whether I'm watching a video, using the web browser, sending an email, editing text... whatever.
I can access via SSH, restart the gdm service and login in again.

The dmesg logs (journalctl reports the same) shows: (each block related to independent sessions)

[code][ 3784.460367] NVRM: GPU at 0000:01:00: GPU-40464710-0928-b0c2-089d-8bfab780f44a
[ 3784.460372] NVRM: Xid (0000:01:00): 12, COCOD 00000001 e0012d00 0000502d 000008fc fffdffff
[ 3784.549068] NVRM: Xid (0000:01:00): 13, 0003 00000000 00008597 0000153c ff000000 0000000c

[ 288.078518] NVRM: GPU at 0000:01:00: GPU-40464710-0928-b0c2-089d-8bfab780f44a
[ 288.078534] NVRM: Xid (0000:01:00): 13, 0003 00000000 00008597 00001080 41000000 0000000c

[ 5458.930150] NVRM: GPU at 0000:01:00: GPU-40464710-0928-b0c2-089d-8bfab780f44a
[ 5458.930154] NVRM: Xid (0000:01:00): 13, 0003 00000000 00008597 00000930 ff000020 0000000c

[ 1697.200550] NVRM: GPU at 0000:01:00: GPU-40464710-0928-b0c2-089d-8bfab780f44a
[ 1697.200558] NVRM: Xid (0000:01:00): 13, 0003 00000000 00008597 00000954 ff000000 0000000c

[ 2292.088881] NVRM: GPU at 0000:01:00: GPU-40464710-0928-b0c2-089d-8bfab780f44a
[ 2292.088891] NVRM: Xid (0000:01:00): 13, 0003 00000000 00008597 00000984 ff000000 0000000c

[ 2024.280013] NVRM: GPU at 0000:01:00: GPU-40464710-0928-b0c2-089d-8bfab780f44a
[ 2024.280031] NVRM: Xid (0000:01:00): 13, 0003 00000000 00008597 000010b0 ff000000 0000000c[/code]

In case I could help devs around here (testing/modifying settings), please tell me.