I got the same problem. there's at least one other gtr 34 i know of with the same fault.
I managed to fix it a while ago by opening up the boost sensor and cleaning the circuit board with cleaner. It worked flawlessly for about 3-4 weeks and only recently has started dropping in and out.
I also have Power FC...do you have the boost controller as well as I was told that was the only way you would see the PFC show boost on the hand controller. I ask because when I first got the car but didnt know much about the hand controller I do recall seeing the boost/vaccum readout. Now I don't but when running a sensor check I get a voltage showing.
I'm going to try and clean the sensor circuit board again and when I get a chance, swap sensors with another 34 (don't think it needs to be GTR) and see if that fixes it so I know what I need to buy.
p.s If you do a search for your topic or something like it, you should find my original post from a few months back