Why don't you try the tech support section of official flyff forums?
The only thing I can think of is that gameguard is having trouble of some sort. I remember a while ago people were getting similar problems, their PCs were just randomly shutting off, and it was gameguards fault.
same thing happend to mine..except every time i watch video's or go to other site...it shutdown like *POOF*..and yea i think try to uninstall flyff might help...and just play flyff on new computer lolz...
Okay your problem is most likely overheating
Flyff uses alot more CPU than say notepad
Your computer will get hotter faster
Laptops are known to heat up easily
Your laptop has a safefy mechanism, once it reaches a dangerous heat, it will immediatly shut down.
I had the same problem on my laptop, so what i did was opened it up and cleaned out all the dust with canned air, and if it is still crashing try having your laptop on a hard surface, most have vents on the bottom.