Platinums are fine if they are the correct heat range. I ran plats in my car for the whole 150k miles, of which the last 80k were boosted, with lots of road course time. Plus, you can generally run a bigger gap with plats compared to coppers, so you should get better gas mileage. I changed them roughly every 40k miles, and even that was a little bit of overkill. If you keep the boost down and only daily drive, you can get away with heat range 6. For more serious driving and more boost, use heat range 7.
The stock temp gauge starts to budge somewhere around 110F, is at 1/3 at 160F, and doesn't move from 170F-220F.
Do you have anything to hook up the to the car to see what it's doing? Techtom? Cable for Nissandatascan? Calumsult? Being able to read O2 voltage, coolant temp from the ECU, MAF voltage, etc, are invaluable in trouble shooting.
I know the car can be a royal pain to get into timing mode, but it can be done. Like I said, search the se-r.net mailing list archives. If anything, just check the TPS and reset the TPS idle memory, it's free to do
Also, did you get the 300zx tt or Nismo radiator caps? (1.2 or 1.3 bar IIRC). They'll improve the cooling system, less chance of overheating.