R9 280x Core Voltage?

Hi, I received two ASUS 280x yesterday to mine litecoins with. Unfortunately, one of them looks to be defective (or I wrecked it?) because when I plug it into any PCI-E slot, windows does not boot and just shows a black screen. I was mining with it very briefly yesterday to test it out and mined for about 15 minutes total, never exceeding 90 celsius on either card and now it doesn't work anymore. I've tried a lot of stuff to try and fix it and will probably try more stuff but the card that does work seems to be getting a wide range of kh/s when mining with cgminer - from 400-700 kh/s.

I think the reason for this could be due to the core voltage of the card. I am using MSI Afterburner to overclock and the default core voltage is 1019mV. I've read other posts of people with 280x where their core voltage is upwards of 1200mV and if I change mine to even 1050, it seems to be fairly stable at 680kh/s.

So, is 1019mV too low of a voltage for a 280x or is this normal? What are other people's core voltages for the 280x?
If 1019mV is too low, can it wreck the card by using it with too low of voltage and may be the cause for my other card not working?

Other info:
- ASUS 280x directcu ii top v2 gpu
- Brand new 1050w Corsair PSU
- Gigabyte Z77X-UD3H motherboard
- i7 3770K processor
- windows 7
- catalyst 13.9

cgminer config:
setx GPU_MAX_ALLOC_PERCENT 100
setx GPU_USE_SYNC_OBJECTS 1
cgminer --scrypt -I 13 -g 2 -w 256 --thread-concurrency 8192 -o stratum+tcp://global.wemineltc.com:3335 -u user -p pass

Thanks.