Do you have a ticket open with GTAC? If not, I highly recommend doing so, as they were great in resolving an issue similar to yours.
Using airport utility inside terminal a Mac.
As mentioned here is what the output looks like from my Macbook Pro, running Sierra, using the airport command inside Terminal. The "-s" flag will show APs and their associated SSIDs and their signal strength. The "-I" flag will show the SSID the Mac is connected to and it's signal strength.
As you can see the output above shows that I'm connected to an AP at -54 signal strength. The closest neighboring AP which is the classroom beside our office is at -85 signal strength. That -85 sits below our threshold, where we have set it at -80 to disassociate. (This maybe different for you)
I had my colleague ping Google on an iMac connected to 5Ghz. Average ping times were about 7ms.
If you have access to the iMac, run those commands and see what output you get. You may have to physically go to the location and measure the signals where you are seeing this problem, and adjust your radios accordingly. That's what I had to do to tweak certain areas.