Well, the system seems stable at least, though I have yet to set the memory settings at their optimum. For now though, I'm just happy it's stable.
I did receive a reply from GGTS - it reads as follows:
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Dear Sir,
Thank you for your kindly mail and inquiry. About the issue you mentioned, unfortunately, if the problem still remains, a further testing or examination to your system might be required. We suggest you to contact with your supplier for checking and repairing. If your supplier cannot help you, please contact with local distributor for further assistance. For their information, please kindly visit our website by following link: CLICK HERE.
At last, if you still have any further question to this issue or suggestion about our products/service, please do not hesitate to contact with us. If you have any issue different from original, make sure you issue a new form in order to get proper assistance and it may be response faster as well. We will try our best to help you resolve the problem ASAP.
Best Regards,
GIGABYTE TECHNOLOGY
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So I'm not really sure where to go from here - I guess taking it back to the retailer (which happens to also be listed as a supplier) and see what, if anything they are able to come up with.
I've seen a few other forums around the web with similar problems, none of which has a solution. It may be that the simplest thing is to get a usb3 addon card.
I'll let you know how I get on.
Thanks for taking the time to read and reply.