Hi Matty,
I'm Jewel with Kingston Technical Support and would like to offer our assistance. I can confirm that our HyperX Fury memory is compatible with the ASUS Sabertooth 990FX R2.0 motherboard. If the system works properly when either one of the memory modules is installed by itself, but not when they are installed together it could be how they are being installed or something going on with one of the slots. According to this Motherboard's manual when installing two modules they should be inserted in the DIMM_A2 and DIMM_B2 slots which are the beige colored ones. If that does not work try them in the matched black colored slots A1 and B1 to see if that makes any difference.
If no change there, check to see what speed the memory is running at in the BIOS. As these are plug and play modules they should be able to run at 1866 as long as your processor supports that speed. If you have any overclocking set up, reset the BIOS to defaults and see if that makes any difference.
For direct assistance, please contact our support team at 1-800-435-0640 (USA and Canada only) M - F 6am - 6pm PT and an available technician will assist you.
If in another region, check here for our support team in your area:
http://www.kingston.com/company/contacts#tech
Thank you for choosing HyperX Fury memory!
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