Tried upping the limits and lowered the clocks by 100Mhz core and 200Mhz mem. It made a difference in that it took longer for the problem to show but it still happened.
I have also now been able to try it in another system, an i7 930 rig with an 800w PSU. Yes the system is a little old but it still works perfectly fine, and yes, the some problem happens. After the first run or two of a benchmark the scores start to plummet:
https://www.3dmark.com/compare/fs/16278166/fs/16278227#
I've contacted Overclockers once again and I am not happy with their response. They are insisting that the card is fine and that there must be something else wrong (they are currently blaming CPU throttling on the second rig as 3DMark did glitch and report the CPU at 4.5GHz for one run, it's running stock btw). They've said I can return it but they will test it again (and have given me no specifics or numbers of any benchmarks) and then return it to me if they think it's ok. I'm really not happy and this has been going on for nearly 3 weeks now.