3.4L Need advice about intake/engine

my problem is that my alero 99 3.4l is kinda lagging from like 2000rpm to 4000rpm my engine as poor torque and from 4000 to 6000rpm i get a fair acceleration ive been trying to find what is the problem i think im gonna have to rebuild my headsbut also for my cold air im using a 3inch tubing should it be smaller ? or if u guys have some more suggestion.. that would help me cause ive cheked like all data are in spec mostly my o2 sensor are working great too im gonna do a comporession test too anyway thx guys

It would help to post your mileage.

3" tubing for a CAI is fine. What brand CAI is it?

K&N cold air millage is about 230 000 km