Apologies for the late reply! I had a gig on saturday night and have only just woken up. =P
I would still say personally that the PSU could be a contributing factor in this. Try using the following website to determine PSU usage:
http://www.extreme.outervision.com/psucalculatorlite.jsp
Your 6300 should be more than enough for handling any of todays games, especially when paired with a 760.
What is your GPU usage when playing B3? If it is high, then there is something amiss and we need to look into it further. If it is low, then maybe you are using bad drivers, OR there is a possibility you ahve a faulty card. If you do have low GPU usage, try what Raheel Hasan suggested and re-install the drivers, (Only after FULLY uninstalling all related drivers) and see if that makes a difference. If it doesnt, then maybe because the 700 series are still new Nvidia still need to fix the bugs in the drivers.