Lately I have had a number of singers come to me with similar problems; the voice is weak on the bottom, loud in the upper middle, and screamy on top. The sound is airy and without focus, frequently out of tune in the passagio area. The throat and jaw are tight, and the sound is uncomfortable to listen to.
This particular complex of problems is often associated with the student trying to “move the breath”. They send the breath forward, which lifts the larynx and puts air in the sound. To me, they are using a sound pedagogical principle the wrong way.
The easiest way to explain what I mean is to illustrate with “blowing air” and with “warming air”. Blowing air is only too easy to demonstrate; just make a loud whisper, like “hah”, and feel the air against your hand positioned in front of your face. You will see right away that the larynx is lifted and the throat contracted.
Now try feeling warm air against your hand, positioned in front of your face without making a sound, as though you were trying to warm your hands with your breath on a cold day You should notice that the larynx is low, the pharynx is expanded, and the palate is high. This was all achieved without trying to lower the larynx, expand the pharynx or lift the palate.
The feeling of “come si parla” or singing like you speak, means there is a clear point of pronunciation in front of the face. Most people feel it either on the hard palate or just in front of it, between the nose and the skin of the nose, or at the bridge of the nose. Christa Ludwig described it as the "point" in the vowel. Because it is a resonance effect, and not an actual place of physical vibration, exactly where you feel it is unique to the singer. You find it by speaking clearly and directly, as in a “demonstration” of clear speech. You can also find it with a quick hum, and identifying where you feel the vibration.
By all means, move the breath when you sing; but use warming air, not blowing air to do it. Aim for a sound with frontal focus and a clarity related to speech. Whatever you do, don’t blow the air past the point of pronunciation!