Your system is super fast...so...
It wonders me because i see people over the net with those problems like you. (i mean it plays wonderful for about one year, and then first time playing again and boom huge frame drops.)
A reason could be the many mod's you have, have been probably updated until now.
Another reason could be that the latest patch may not have probably updated your older patch one year ago. (for example 1.9 patch over 1.4 patch)
I assume you have the latest unofficial patches installed, but i don't know really if that explains the huge frame-rate drops.
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Actually there is one workaround to see what's going on....restart with an new character !!!!! (first with all mod's disabled and then with all mod's on.)
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The above are general talking....but if you changed you graphics drivers to the latest one, and sure you did, perhaps it's a drivers fault as my friend CCNA said above.
Me speaking, i haven't encounter any strange behavior with the latest Nvidia drivers (331.65), considering i play on Windows 8.1.
But again, i haven't any mod(s) installed !!!!!!