Try this:
-setx GPU_MAX_ALLOC_PERCENT 100
-setx GPU_USE_SYNC_OBJECTS 1
cgminer --scrypt -o -u -p -I 20 --thread-concurrency 8191 --worksize 256 -g 2 --lookup-gap 0
840 with a lot of hw error.
I don't know 100% the blue screen is related to the tc. When i play with cgminer (start and stopping) the driver may stop responding and I need to reboot and the occasional bluescreen. But if I reboot and start cgminer and let it run = no problems.
Oh my bad, try that with higher TC...was a bit sleepy. :p