(Reposted from the legacy Sonar forum)
Here's some free advice, in case anyone else ever experiences the same issue I just had. After spending several months tuning X2 on my custom-built DAW, I ran stable for a while, but suddenly started experiencing audio dropouts about a month ago. CPU was not stressed (i7 3.0 Ghz) running at about 15% under load, RAM was plenty (12 GB) at 60% free even with 5 instances of Kontakt 5, Windows 8 OS with nothing but DAW software running, save for NOD32 which has a very light footprint.
I tried everything under the sun to try and resolve the issue - nothing. I finally concluded that I needed to swap out my 7400RPM audio drive and, though I intended to purchase a 10000RPM Raptor, I went and bought an SSD instead. While driving back, I noticed an audio dropout thread on a Cubase forum (of all places!) in which someone - after almost a year of debugging - realized that his USB-powered controller was the cause of his dropouts when the machine was over a given threshold of activity. I ran home to check and, sure enough, my Roland PCR800 power adapter was unplugged, which means the controller was sucking USB power instead. As soon as I plugged the power back in, dropouts were gone!
Note to the wise out there: Do NOT run USB-powered external gear on your DAW. You could get very unexpected results - including inexplicable audio dropouts!