I would also think ignition timing with header primaries being "red" or fuel pump not keeping up under load at RPMs. Check your wiring/ground/connections to the pump because that can greatly affect the pump operation as well. Something else worth checking is your camshaft timing components. If the oil pressure activated chain tensioner is not working properly it will be changing your cam timing at different RPMs which in turn changes your combustion/fuel/spark/power...just a thought. I had a chain tensioner blow a plug out that looks similar to a freeze plug once and experienced similar problems. I had fun trying to diagnose until I went to the dyno chasing other ideas and found the problem. I bought a new one from Nissan and had the plug tig welded in place and have had no more issues. Ofcourse I fished the old plug out of the oil pan.... Good Luck. Your best bet is to make friends with a dyno owner!