My engine cuts out under load in 3rd 4th and 5th gear

[SR 510]

OK, Here's what going on. I have a Datsun 510 with an SR20DE. But, ever since I swapped it in my car I have been having a strange problem. The motor runs really nice and has plenty of power, but everything cuts out and bogs when I am at 6,000 RPMs in third gear, 5,000 RPMs in fourth and 110mph in fifth. When it cuts I cannot accelerate for about 5 seconds and then the motor runs again. The strange thing is that when I am going downhill I can go 125+ MPH and when I go up hill I can't even get it above 100 mph. I am guessing it has something to do with load but I am completely stumped. If somebody could help me I would really appreciate it. Thanks

note: I know that it is not a speed limiter because I have no wires running from my transmission to tell the computer what gear I am in, nor how fast I am going. I have a walbro 255 inline fuel pump. I also have huge custom fuel lines and bigger inlet/outlet from the tank and My fuel filter is fine.

-Keith

[RED_DET]

Have you checked MAF ground, TPS voltage, etc? What ECU are you using?