1990_Rb20_240

Okay so i have an rb20 in my S13! Now i dont have a working tach but i can tell you where the car cuts off by the MPH! Now the problem with my car is kinda a strange one, when i drive it the car runs great when you drive normal but its when you floor it that it starts acting wierd! When i floor it as im approaching 25 mph the boost begins ton fall off and then when i reach 25 it just falls on its face and i cant go any further than that cuz my engine sounds like its gonna blow! And it does that at 45 mph in second and 65 mph in third then like 90 mph in fourth!! I dont get what the hell its doing???

I replaced the feul pump with a walbro 255 i put new spark plugs in i checked for boost leaks and i dont have any!! What is the deal??