You do realize that at those RPMs, you are in the peak cylinder pressure range, which is right where spark blowout can start happening, even at low boost. This blowout causes misfires, which in turn causes unburned fuel and air, which the sensor will read as lean. If you have anything else going on, it can make it difficult to sort out what is going on. What is your spark gap set to?
Also, what VAM do you have? It is possible that you might have a bad VAM, and it is not reading flow accurately, so the computer is not adding fuel accordingly. You might have to sweep the VAM though its range and make sure it doesn't have any dead spots and that its not sticking.
Just a few things to check, I know figuring out stumbling and such can be really tricky.
Another thing to consider, and its hard to check for, is that if a few valve springs are getting weak, it is possible that at higher RPM that your valves are floating. If this happens, even though you boost is high, your actual cylinder pressures would be low. Just remember, if anything that happens to prevent complete combustion occurs, then you get unused oxygen too, and the O2 sensor or wideband sensor cannot tell the difference between an unburned mix and a lean mix.
I'm not sure if you year has a knock sensor, it is also possible that you are getting knock, and that the timing is being pulled in that range, which will also change the wideband readings, and will show up as a stumble or hesitation. Don't disable the knock sensor to overcome the issue, instead try to figure out how to stock the knock, you might need to pull a degree or two of timing in that part of the timing map.