Anyone got any ideas or suggestion / help for these please: 

I Have 2 x XFX R9-280x-TDFD / R9 280X 1000M BOOST 3GB D5

They have been clocking very stable at ~702Kh/s for 3 weeks ( GPU clock 1020, MEM clock 1500, 1200mv )
powertune 20. Using CGminer3.7.2 each card matching pretty much constant.

I have also had them run stable for over the last week at this ~725Kh/s ( GPU clock 1050, MEM clock1500, 1200mv )

Just the last 2 days one card has been dropping down to ~614Kh/s regardless of clock settings above and into the ~500KH/s at stock clock of GPU 1000, MEM 1500

It starts of ok for the first 15 mins where they both match then it drops off bouncing from 64% load to 99%load spending like most time between 64% and 70% load. approx. 580kh/s  - 615Kh/s THE OTHER CARD JUTS SITS STABLE 724Kh/s - 725Kh/s

I have ran several programs to check it, the only thing I have noticed during the tests and overclock programs is that the fan speeds reports they are only spinning the fan at 70% of the other card even when it reports 100% in test programs it shows actual rpm of the other cards saying 70%.

So while 1 can spin 3150RPM at 85%     the other card reports 2800RPM at 100%

OR there is a bit of give and take on the figures below +/_  of 25 - 50  I just took averages.

GPU0 Fan speed ~100% =RPM 3150   GPU0 Fan speed ~85% =RPM 3150   GPU0 Fan speed ~75% = RPM 2850        
GPU1 Fan speed ~100% =RPM 2800   GPU1 Fan speed ~85% =RPM 2800   GPU1 Fan speed ~75% = RPM 2500

I have swapped the cards round in there pcie x16 sockets and it makes no difference.

I have tried running in both CGminer 3.7.2 and SGminer4.0

In both programs the same card is down in KH/s on what it normally is, while the other runs same as always. They are only 4 weeks old.

Any Ideas, advice  or suggestions would be welcomed ?

Thanks Axco