I have a 4-unit mesh of M5s, interconnected with fixed Ethernet. My network was mixed 2.4GHz and 5GHz and have now switched to 5GHz with no improvement in network performance. Here is the issue:

All my wifi clients grind to a halt for a minute or two. If I ping the IP of the AP directly, I get consistent replies of a few milliseconds. But if I ping another client connected to the same AP, I get repsonses with very high jitter (hundreds of ms). This is repeatable across several devices attached to the same AP. 

I don't get the same issue if I ping a device attached to the Ethernet port from the wireless side (and as above, also not if I ping the AP's IP). This leaves only one conclusion: the AP cannot properly bridge frames between directly attached clients. If it were a radio issue, I would get the same jitter regardless of destination.

All APs are set to bridging mode as I have an external router providing the default gateway function.

Another wireless device attached to the same AP

The default gateway attached via Wifi -> Ethernet