Even in a deployment as small as this one (they only had eight APs) using 80MHz channel is a non-starter. The WLAN was literally interfering with itself. Even enabling DFS channels would have been a difficult thing to make work, even if (theoretically) we could have SIX channels. However, channel 144 is ONLY usable by 802.11ac devices. So, typically, the 80MHz channel that includes 144 is not used. Also, the TDWR (Terminal Doppler Weather Radar) channels are not used so now we're down to FOUR 80MHz channels. Four channels may work in a small environment, but then you have to consider WLANs other than yours.
Unless you are in a secluded area there will probably be other WLANs within earshot of your network. So, if any of the sub-channels that make up the 80MHz channel that you are on, are also in use by other clients, you will experience issues. In this case, the WLAN was in an office building that had other tenants with their own WLANs. So, using 80MHz channels in the scenario was the main cause of their problems. (Even if they were secluded usage of 80MHz channels would not be practical.)
The fix was simple. We enabled DFS on their controller and disabled 80MHz support in the ARM profile for that AP group. This immediately dropped all APs (access points) to using 40MHz channels. And, with the use of the now available DFS channels, we had NINE channels to use among 8 APs. The point here is DFS gives A LOT more channels to work with - so it's worth using them.
We can't blame the customer for this. The installer should have known that 80MHz channels in an enterprise environment is not practical, or in this case, even viable. It was clear what had happened. The system defaults were left as-is, and there was little to no customization of the WLAN.
By default, 80MHz channels were enabled.
By default, DFS was disabled.
By default, all APs were set to transmit at maximum power.
This just shows the importance of knowledgable, experienced professionals deploying your wireless network. It's more than mounting APs and creating a few SSIDs. It's understanding how RF (radio frequencies) work, what its limitations are, and how to design around them.
One last note on using DFS channels. I have seen great success using them, but the reason DFS channels are not enabled by default is because they share spectrum with radar systems: