Weird Media Bridge Mode issues

I have two RT-AC66Us, one in Router mode, one in Media Bridge mode. They connect to each other on the 5ghz band. At least once a day, the media bridge and most of the devices connected to it will drop from my network. If I go to the Router's web admin interface, and look at the connected devices, the Media Bridge will not be listed there and I won't be able to ping it or access it's web interface. However, two of the devices that are connected to the Media Bridge are still listed there (an IP phone and a server), even when the Media Bridge drops from the network. What's even weirder (to me, at least) is that the server that never drops is actually a virtual machine on another server that does get dropped from the network whenever the Media Bridge "disappears". Clearly, the media bridge is still connected, since i can access two of the wired devices connected to it, but I can access the Media Bridge itself, nor some of the other wired devices connected to it. I'm using the latest firmware (had same problems with previous firmware too) and have tried switching which router is in Router mode and which is in Media Bridge. Any ideas what is going on? Do I have a defective router or two, or is this a known issue?

Thanks in advance!