BFG Miner under Win 8 Keeps Crashing

[No_2]

I'm using BFG Miner under Windows 8, mining with an HD 6990 and it keeps crashing every 30 or so minutes.

Can anyone suggest what may be the issue?

I also cannot install CG Miner onto windows 8 which seems to run fine on my other rig in Windows 7 Home Edition.

In both instances I am mining LTC on coinotron.

[No_2]

BFG Miner is reporting the following error:

    OCL 1: Invalid Nonce - HW Error

Running BFG - with these settings:

    C:\Users\Miner\Downloads\bfgminer-2.99.1-win64\bfgminer-2.99.1-win64\bfgminer.exe -o stratum+tcp://coinotron.com:3334 -u [username and password removed] --scrypt --thread-concurrency 8192 -I 15,14 -g 1 -w 128 --temp-target 85,85 --temp-overheat 95,92 --temp-cutoff 105,102 --auto-gpu --auto-fan

[No_2]

This seems to have been fixed (for now) by increasing the voltage - can anyone explain if this is a likely cause and why this is?