Random crashing - extremely often

[Aerycat]

Details:
Random crashes. I just started not long (2-3days), and been only around in Gridania. Few symptoms :-
1) Crash happens with APPCRASH - Detailed log will be posted later.
2) Computer just reboots halfway playing. On one occassion, I was monitoring both CPU / GPU temp and GPU is at 80c and CPU at 61c when the system just rebooted.

Steps:
Completely random. No trends observed.

Date & Time:
Random as well.

Frequency:
Once in about 20-30 minutes of running around, or even staying put in one place (idling).

Platform/OS:
Windows 7 64-bit

PC Manufacturer & Model Number:
Homebuilt

Graphics Card:
ASUS HD 6990

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Things I've tried on my side :-
- Did a stress test on GPU / CPU both reaching 90 degrees without problems observed. (Stress test done on furmark / prime95, to rule out high temp / power instabilities.)
- Tried multiple settings on FFXIVConfig (Window or fullscreen) still the same.
- Disabled DWM - still the same.
- Reinstalling the client.
- Reinstalling the OS.