OK, drivers did need update.
In the meantime I stumbled upon the Client Detail tab of Airwave, and I'm drooling... if only I knew.
I zoomed in on the client, and can see in her avg. SNR chart that she's been weak, also that her laptop's been bypassing the 105, holding on to the 225 down the hall.
So at least I can measure the issue.
Going to remove the 105 and try a 225, my only fear is that it will cause a problem for different users.