This is the issue I have been experiencing and described in an earlier post on this thread - post #54 on page 3. There is nothing that will fix it other than setting 5Ghz bandwidth to 40 Mhz. You can not use 80Mhz or 20/40/80 Mhz settings. Factory resets, clearing NVRAM, reloading firmware, doing fresh configurations, etc all have been tried by many people and do not work. Only using 40 Mhz channel widths (regardless of channel number chosen) fixes the issue.
I wish either Asus or Broadcom would acknowledge this fault and hopefully produce a version of firmware that fixes it. The older Asus 3941 code seems to be the only original Asus firmware that has stable 5Ghz WiFi but it is not Merlin code and he doesn't have a version directly based on this code. I know people on the boards have tried Merlin 380.62, 380.63, 380.64, 380.65, and 380.66 and all still have this 5Ghz problem. I too have used all of them to no avail. I have been using 380.67 Alphas and now the 380.67 Beta code and the problem still persists. I have my router on 40Mhz channel width and all is fine as it was with all other previous Merlin builds.