Re: my system
If both cards are the same GPU and same amount of memory on the cards it's highly unlikely that its the GPU's.
First thing I would try is running with only 2GB of memory. Using 3 slots on a Dual-channel motherboard can give some strange results.
4GB of memory really is preferable over 2GB for SLI.
MSI MPower Z87 | 4770k | CNPS9900MAX-B | 8GB G.Skill 1866 | EVGA GTX760 |ThermalTake 850w | X-Fi Titanium | HAF | Asus PA248Q | Lycosa | Death Adder | Win8 Pro 64
MSI GD65 | 2500k | H50 | 8GB G.skill | GTX570 | Ubuntu 13.10
17" Inspiron | i5 | 4GB DDR3 | 500GB HDD | Win7 Pro 64