I haven't installed the beta firmwares.  It is possible this helps, but we are also experiencing the issue with security appliances with built-in wireless.  Unsure if the beta MX firmwares include the same updates. ... View more

Also if you're only using one antenna make sure you are not running dual band on these AP's. And to avoid moisture problems get waterproof metal caps to cover your open N-type ports. ... View more