Does your ram have XMP ? (Very easy to set it to 1600mhz if so) although 1600mhz won't be guaranteed, you won't know until you try
1333mhz is the jedec standard for ddr3, by default that is what it will run at unless you configure the timings and speed, XMP profiles allow this to be configured by a profile. But without setting it, it will go to the default settings. If you have stability issues with the ram at its XMP profile it could be a issue with the ram, Some sort of memtest would be the best confirmation, I use ultimate boot cd and run the test from there.
