Q: Wifi connection constantly dropping on mid-2010 MB Air?

My wife's MBA is constantly stalling trying to load internet pages.  Her connection to our home network seems fine at times and then, in the same location, with all 4 bars of signal strength, just seems to stop working.  When she looks under the Wifi menu drop down, she will see our Airport network, but often can't even see the Verizon router network.  Turning Wifi off and back on seems both reconnect the network and allow her to load pages and allows her to see the Verizon network.  We have a mid-2010 MBA and late-2013 MBPr that are connected to the same home network using an Airport Extreme with Wifi ac (the same problem was present when we were using our previous Airport Extreme with Wifi n).  Our primary (listed first in Network > Advanced) network is the Airport Extreme which is a separate (not extended) network in addition to the Verizon Fios n router which is our backup/secondary.  Both computers are on 10.9.5 and the router has most up to date software.  I have tried upgrading her hard drive from 256 to 512 (she was running out of space and computer seemed slower in general) which did nothing.  I have rebuilt the desktop w/ Disk Warrior repeatedly, repaired permission and repaired disk.  I'm wondering if there are pref files or other software related things to try or there are known issues with MBA wifi connectivity.  Seems this problem did not occur for the first year or two of owning the computer and it has gotten progressively worse over time, making the computer almost unusable at times.  My MBPr is blazing fast and does not have this issue, only occasionally having problems connecting to Wifi when waking from sleep, which I think is a known issue and not that big a deal.  Can anyone offer any suggestions on next steps for software fixes and/or thoughts on how to determine if there is a hardware problem?  Thanks.