Update:

One of the cards has now dropped from 0.75 Mh/s to 0.70 Mh/s in about 10 minutes, the card was running at 72-75 degrees stable before the hash-rate drop started occuring, it has since been dropping its fansto stay at 70+ degrees.

Clocks, and load remains the same, and the rig is pulling 850W from the wall socket.

Update 2:

Now down to 0.68Mh/s and slowly dropping more... Clocks and load remain the same..

Update 3
Another 10 minutes has gone, in Gpumon the card shows a value of 925.5K/652.1Kh/s and keeps dropping, the 'K' value of the culprit card is higher than that of the other cards, where it usually matches the Kh/s rate

Also tried resetting it resulting in an error in cgminer:

[14:33] GPU1: Idle for more than 60 seconds, declaring SICK!
[14:33] GPU still Showing activity suggesting a hard hang.
[14:33] Will not attempt to auto-restart it

Tried disabling miner, Gpumon/web-dashboard shows 99% load even when mining is turned off, now rebooting the machine.

Now trying the affected card at 1060Mhz to see if it consumes to much power to keep the earlier 1070Mhz clock, Hashrates so far seems almost unaffected, but the invalid rate has gone up, its only been running for 5 mins or so, but 2 invalids already..