I have just sold my 89 GTS-T Coupe which had just replaced my 84 GTS-T Coupe. I had to replace the front bushes twice over 3 years & approx 120 000 km's. The other failures I had were the auto which blew itself to pieces, the oil gauge failed (fitted a 3rd party add on after the failure) & the electric window motor on the drivers side. The good points of this car were many... The handling was superb, it was fiited with NISMO suspension when I purchased it & the power was good old kick you in the pants material. They are a great car to drive, well balanced with good power on tap. I have to admit that after selling my old one a couple of months ago I am now looking out for a later model as I miss the way they drive.
A little late with my comment, but here goes the help... first I must assume it is standard, thus fuel and boost can't really be at fault. I can however offer the following 1. If an import check computer for any messing around, Japanese like to play with even the simplest of thing that have devasting effects on crappy RON's (fuel) here in Aus/NZ. You may find it has been.
2. It's unlikely that if it has been missing at higher revs/load there can be much more than an issue with fuel or EMS, I suggest you chat to someone who specially deals in Nissans full stop. A professional who can accurately find problems on a dyno can offer a lot more expertise than some comments on a site like this. Find the best dyno operator you can, remember you must exactly know the problem (RPM e.t.c) when the problem exists saving you $$