Couple of things to do/check...
1) Change the fuel filter...
2) Never put in 91 octane, try for 94
3) Your boost is kind of high... you risk blowing #6 Cylinder... (common)
4) Reset your ECU, it may have been tuned for more octane.
This section has a lot of helpful info... see the PDF's...
http://www.jdm-imports.com/skyline_specs.html