After reading another persons issues that sound similar to this, i'd check your RAM settings in the BIOS. Make sure you are at the correct frequency, voltage and definitely make sure the RAM timings are correct.
 

CptSpig
Saying that the ACX coolers don't cool as well as reference cards is so untrue. ACX cards stay much cooler than reference cards this has been my experience. If you have a problem with these cards it is usually the case does not vent properly. But this is really a moot point because it looks like it's a driver issue. One last thing did you use display driver sweeper or some program make sure the entire driver was uninstalled? If not I highly recommend you do this and than install the working driver again.

arestavo
Ambient temperatures can play big a role as well, causing the GPU to hit the thermal throttle threshold.
 
One way to see is to take the side of your case off and check it again. With older even hotter cards like the 480s, I had to put a house fan blowing in on the cards to keep them cool. That shouldn't be the case with a 780 Ti, but you have the ACX version which dumps most of its hot air back into the computer case, and doesn't exhaust all of it like the reference 780 Ti's that I have (which is the reason why I will never buy EVGA's ACX cards due to my preference of using two cards and the inherent heating issues that can cause with ACX coolers).

Vanilla cards are built to be sandwiched together in SLI, they work better than the top dual fan cards because of the design. A single dual fan card is going to be better than a vanilla because the cooling is better and the amount of heat that it dumps into the case isn't going to effect that much. But when you are running more than one video card in the case, it dumps a lot more heat into the case and makes one card run about 10C hotter than the other... where the vanilla cards have a smaller temperature gap between cards and less thermal throttling.

I had a similar issue with my new Titan. Try uninstalling Afterburner/Riva Tuner or Precision X totally.  Say NO to saving profiles. 
 
Use DDU to get rid of the driver.  Install the new driver only checking Phys X. Uncheck everything else.
 
 Use Advanced then Clean install to find these options.  DO NOT install GeForce Experience.  Uninstall it if you have it.
 
Test you card after doing all of this.  It worked for me. 
 
Good luck!