your assumption that it's going extremely lean may be a bad one?

consider that a gigantic series of misfires of some kind will show up as really lean on a wideband (your wideband probably maxes at 22:1 or something, it's probably reading pure air).

your ignition could be breaking up in a certain range or something, or something else might be completely messed up, hell you could have some kind of mechanical issue going on too

it makes no sense the injectors would fail to fuel mid-range then suddenly start fueling again at high rpm. you'd have to have an insanely large spike in volumetric efficiency in that 3000-3500rpm range -- engines generally move more air and hence require more fuel as RPM rises.