Dooes Windows load any background programs? There's a common misconception that streaming from disk means you don't need much RAM, but Kontakt stores a significant amount of a note's attack so it can play back instantly while the hard drive seeks. If there are lots of individual samples for articulations and such, the needed memory can add up fast.
Your saying that it crashes about half the time may mean there is something about your system that's "on the edge." I don't know if X3e loads more "stuff" into memory than X2; if you ran programs prior to opening Sonar, sometimes they don't release memory fully. If Sonar had a bug caused by loading large files, it would seem that it would happen every time. If Sonar can load files smaller than a certain size without errors, it would seem whatever the problem is relates to file size.
When I bought a new music computer several years back, I mentioned that I use Kontakt a lot and the system integrator (PC Audio Labs) highly recommended 32GB of RAM. At the time I thought that was overkill, but it turned out they were right. I never have crashes any more from running out of memory.
Finally, I've found NI's support very helpful in the past although it can take them a few days to respond. They really depend on lots of data, so for example try running Kontakt in standalone mode to see if that makes a difference. If it's fine, then open some more instruments. If you reach a point where you try to load so much stuff Kontakt crashes and you get the same error message, that would likely indicate a need for more memory. My understanding is that Windows isn't all that efficient in terms of managing memory, but that may just be something I recall from conversations with Mac-centric friends
