2001 Nissan Sentra Engine Problems

I got a 01 nissan sentra se 2.0l with 5sp.
it had a bad engine and we replaced it with another one. The problem is that when you are accelarate at full throttle, in the range of 2k-3k rpm, engine starts to shake, or stutter. It feels like its not getting enough air or too much of it, or even too much fuel. Now if you accelerate smoothly (without stepping on it) it seems to be doing just fine. I tried replacing different sensors and gadgets from the old engine and its still the same, tried replacing spark plugs, wires, distrubitor cap, and its still the same. The check engine light is NOT on. There is no smoke in the exhaust.
any ideas?
thanks.