Raspberry Pi 3 loosing wifi - busy status

[Nachteule]

So, did not fixed my issue with onboard wlan device. After changing wlan channel it is more stable but still not perfect

Your issue may be related to wlan power saving.
You could try disabling power management by command

Terminal

sudo iw wlan0 set power_save off

or adding one line to /etc/network/interfaces (I did this on my RPi3 now)

Code:

-- snip --
iface wlan0 inet static
       address 192.168.1.24
       netmask 255.255.255.0
       network 192.168.1.0
       broadcast 192.168.1.255
       gateway 192.168.1.10
       wireless-power off            # <-- Disabling power management
-- snap --

Seems not to be kernel issue > 4.4.12 and not firmware issue. Reverted back to older versions but did not make any difference.

HTH