After so many days of complaints from many users in this thread.
Devices tested on 5Ghz:
Sony Xperia Z1
<<< NO issue on Wifi
XiaoMi Tv box
<<< NO issue on Wifi
Sony Xperia Z3
<<< NO issue on Wifi
Samsung Note 3
<<< NO issue on Wifi
Samsung S5
<<< NO issue on Wifi
ASUS PCE-AC68
<<< NO issue on Wi-fi
Intel AC7260
<<< NO issue on Wi-fi
Apple Macbook Air 2013 - 10.10.2 Pre-release beta to resolve Wi-Fi, but got issue.
Apple Iphone 6 Plus - Build 8.1.2 (12B400)
Apple Iphone 6 - Build 8.1.2 (12B440)
Apple Iphone 5 - Build 8.1.2 (12B440)
Tested router:
ASUS RT-AC87U << 5Ghz drop only on Apple product
ASUS RT-AC68U << 5Ghz drop and sometimes auto reboot only on Apple product
ASUS RT-AC56S << 5Ghz drop on Apple product
Linksys WRT1900AC << Auto reboot and wireless drop on Apple product
D-link Dir 820L << 5Ghz drop on Apple product
Aztech 7003 <<< 5Ghz speed drop to 6Mb and cannot connect on 149 or 161 channel for Iphone
It's seem like apple product is the cause. Apple just released another beta firmware yesterday on MacBook Air to fix Wi-Fi issue.
Google search and found out it's a world wide issue on iPhone wireless drop.