1994 Nissan Sentra 94 Sentra lacking power at mid RPMs high

0k my sister has a 94 sentra took it to have emissions done and passed at idle but failed at cuse limit was 270 and failed with 1377 I have changed air, fuel, plugs, wires, cap, rotor, checked spark, fuel, and checked for codes NO codes. When the car is in gear you can rev it up and their is No problem then took the car for a drive and to low rpm just fine then it gets to mid rpm and is really slugish then get in the higher rpm and then is fine again I have checked fuel pressure under a load and it holds around 40 psi I dont know what else to do please help thanks