Multiple card set-up gpu load

[juve4v]

 I rig with Asrock 970 extreme4. pcie1  1x 5870 on extender 1x->16x, pcie2 1x7950, pcie3 1 x5850-on extender 1x->16x and pcie5 1 x 5970 all powered by and AX 1200 W.
Problem: I start cgminer/phoenix -anything is the same- and gpu usage is at 98-99% but the usage is dropping down  on all gpus by time.For example all gpus drop from 98-99 % to 95% in 1 h, so like 3-4 % / h down loosing this way my hash power.
Untill I added the 7950 never experienced such a problem with the other 5xxx.I suspect a driver problem as I also noticed spikes in cpu usage as well, but I tried all drivers from 12.3 to beta 13.3 with all sdks from 2.4 to 2.8 in vain .
Power is NOT an issue here as they all suck max 850 W according to my power meter.
OC is also not an issue as same thing is happening regardless of OC.
Temps  between 58-65, so heat is not an issue.
Priority of cgminer is set to realtime and monitored gpu load via gpu-z and MSI AB.
Any suggestions?

[1715168800]

1715168800

[1715168800]

1715168800

[crashoveride54902]

keep all 5xxx and 6xxx series card in one machine and all 7xxx series cards in another...drivers don't like 5xxx n 7xxx mixed

[vdragon]

TRue, 7 series are not really friendly with other series cards