Hey guys I have 2 GTX 760 FTW with 4GB each and I'm having issues when playing.  At first FPS are ok, but after a short while the start oscillating up and down.  Also I feel sound lag.  not sure what causes this to happen.  I have huge EVGA 1300 PSU.
 
My board is a EVGA z87 and I have my PSU connected to it through the 2 CPU 8 Pin connectors, the auxiliary PCI-E.
 
Both Cards are connected with their 6 and 8 pin.
 
Please Help. 

I have!!!, I tried them single at lane 1.  I have switched them position 1-2, 1-3, 2-4, 1-4 and no luck far. 
 
Lately I switched to 1-3 to get 8x/8x...Works more stable and game performance has improved however sometimes 1 of the cards won't start.  It's weird though cause when it boots the leds for lanes 1-3 light up green and after bios only one of them lights up red.  So far lane 3 always is culprit.  Lane 1 always starts.  When both cards start both leds switch to red.
 
I'm pretty sure my cards are ok, I tried them both single at lane 1 at 16x and they perform nicely.
 
Please help me out.  Thank you,