schwaps
I am having the EXACT same issue with my GTX 750 Ti FTW when using Precision X.  In my case, it occurs when I have Precision X open while playing COD4 MW1 with overclocking (+ 62 GPU, + 300 memory).  Within a minute or so of switching back and forth to the desktop to view Precision X while playing the game, my GPU clock becomes "frozen" at 405 MHz with terrible frame rates.  The only remedy I have found to work to restore the GPU is to reboot the machine.  However, if I DO NOT open Precision X while playing the game, the GPU appears to be running correctly per the overclock settings.  All indications point to some sort of a conflict either within Precision X or between Precision X and the NVIDIA driver.  (I am using BETA 337.50).
 
Now we know this issue is relevant and is occurring for multiple users with GTX 750 Ti FTW.  The hardware is magnificent, but this issue leaves us with a bad taste in our mouths.  Please help!

schwaps
I just installed the card last evening and decided not to burn all the midnight oil at once, however, it seems to possibly be tied to overclocking the memory, but I am not 100% certain at this time and it may involve GPU overclocking as well.
 
I do have the overclocks set to apply upon boot.  I verified the overclocks are being applied through both Precision-X and GPU-Z.
 
I did try to run factory (default) overclocks with no memory overclock and I did not experience the issue.  However, as I stated, if I keep the overclocks (+62 GPU, +300 memory) but DO NOT open Precision X, I do not experience the terrible fps's during gaming, so I am certain that it has to do with running Precision X simultaneously with my game title (COD4MW1) and it appears some other users' game titles.  Once the clock is stuck at 405MHz, it is stuck at that limit until I reboot.
 
As a side note:  I believe my overclocks are reasonable as the competing product (PNY 750 Ti) is overclocked to 1202 MHz GPU base with 1502 MHz memory while having a much smaller cooling unit employing only a single fan.  I am having a hard time believing that the hardware would be unstable at this particular overclock with the low designed TDP and ACX dual-fan cooling solution.  Plus, my temps are staying in the 30-40's range.  This, together with the well documented similar (and successful) overclocks on the net leads me to believe it is not an instability issue.  And, since the issue ONLY seems to occur with Precision X open simultaneously, there has to be some sort of correlation between that software suite and the game title(s) and/or NVIDIA driver(s).  For example, perhaps when Precision-X is open while the game title is running it is flagging the memory and/or GPU overclock as an instability issue and downclocking when in fact it is still stable?
 
I believe this issue should be fairly easy to reproduce in a lab setting.
 
Thank you for your assistance and for providing such a great product!

schwaps
Thanks again for your response.  Your points are all valid and well taken, but note that the originator of this thread (th3franz) was having this issue with only a 100 MHz (200 MHz effective) overclock of the memory.  I also had similar results with lower memory overclocks, but may need to do further testing to pin it down.
 
Could you kindly elaborate on your first point, "It could be as simple as adding in the extra processing and code of displaying the overlay on top of the game is causing it."  I am taking this as meaning that the Precision X software is somewhat mildly GPU-intensive and, compounded with a moderately GPU-intensive game such as COD4MW1, is pushing the card to instability?  Is this more-or-less what you are speculating?  And, if this is the case, then surely a more GPU-intensive game would produce the same result, with or without Precision-X open?
 
And, taking this a little further, if the memory overclock is to blame, it seems to me that this would indicate that this board (EVGA 750 Ti FTW w/ ACX Cooling) should not have it's memory overclocked whatsoever, despite the numerous reports of successful 600+ MHz memory overclocks abounding on the internet for this particular board.
 
All of this leaves me simply perplexed....  I still think this is related to something within the Precision X application and really want to find a solution.  Unless this can be pinned down, I may have to resign to sticking with the stock memory speed and experimenting with overclocking only the GPU which would be a real downer as I bought this card due to its great overclocking potential.