240koukiSX

Ok guys I'd really appreciate some help on this. The car is a 97 kouki with an S15 SR. I just got it tuned and the car has run great and makes really good power. The problem is that sometimes right at 5K rpms it'll stop making power and act like it's hitting a rev limiter. The rpms will drop and it'll make a lot of power but then hit 5k-5500 and lose it again. It only does it if you go from lower rpms to pegging it at WOT. It drives perfect under all other driving conditions and only cuts out sometimes not every time. I'm trying to figure out where to start and what possibilities I can rule out so I don't waste money. As always any help is super appreciated and thanks a lot!