Diagnosis
Yeah, this one's common. The 5GHz band is likely getting interference or hitting a firmware bug related to channel selection or the Smart Connect feature.
How to Fix It
Log into the router and go to Wireless > Professional tab for the 5GHz band. Disable 'Roaming assistant'. More importantly, change the 'Control Channel' from 'Auto' to a fixed channel, like 36 or 149. Apply the settings. This stops the router from aggressively hopping channels, which can kick devices off.
If That Didn't Work
Turn off 'Smart Connect' (the feature that combines 2.4GHz and 5GHz SSIDs). Under Wireless > General, give your 5GHz network its own unique name and password. This forces devices to stick to the 5GHz band and eliminates the handoff logic as a culprit.
Heads Up
If you're on an older firmware, updating to the latest from ASUS's support site can fix a lot of these stability issues. Just download the file and upload it in the Administration > Firmware Upgrade section of the router interface.