what are the temperatures of the cards and the cpu? the extra load and heat of the new card could be making things unstable. remove the overclock on your cpu, the extra load on the cpu from the new graphics card could cause the overclock to be unstable.
try a clean windows install too, that often fixes a range of problems.
According to GPU-Z, the top card is 36*C and the bottom card is 32*C idle with a minor overclock to test for stability. At 4.5 GHZ, my CPU is running at 33*C idle. My case has excellent airflow so there should be no reason why my graphics cards are overheating. I calculated the power supply load on Thermaltake and the requirement is good enough for a 750W.
You can have 2 million watt PSU and yet have power restrictions. Depending on the cpu power connector, 4/8pin you get a certain ammount of watts on the motherboard (some boards also have an extra molex connector for exactly this purpose). Also, depending on the pci-e power connector (6/8pin), more or less power is drawn from the motherboard. I dont have the charts in my head. But there is a limit.
I have an 850w psu and the CPU overclock I could reach with 1 GPU was higher than with 2. At the time 2 HD 6870´s.