I examined the system logs when it happened again today, sometimes there is something in the logs, sometimes there isn't.
Unfortunately, this is all I can get.
Jun 25 04:52:00 Qosmio-X300 kernel: [ 3370.969985] NVRM: Xid (0000:01:00): 8, Channel 00000003
Jun 25 04:52:01 Qosmio-X300 kernel: [ 3371.506916] NVRM: GPU at 0000:01:00.0 has fallen off the bus.
Model: GeForce 9800M GTS
GPU UUID: GPU-4be018df-46e9-fa95-a270-f018cdbe85e3
Video BIOS: 62.94.6e.00.18
Bus Type: PCI-E
DMA Size: 40 bits
DMA Mask: 0xffffffffff
Bus Location: 0000:01.00.0