Raspberrypi 3 Looses wlan0 after sometime

I am Running a raspberrypi 3 as a small Squid Proxy. but from time to time I need to log in and need to restart the wlan0 interface. The raspberrypi lost its network connection. I cannot reach the box using ping or ssh.

Network configuration /etc/network/interfaces

auto lo
iface lo inet loopback

iface eth0 inet static
    address 192.168.0.14
    netmask 255.255.255.0
    gateway 192.168.0.1
    dns-nameserver 192.168.0.1

auto wlan0
allow-hotplug wlan0
iface wlan0 inet static
    address 192.168.0.15
    netmask 255.255.255.0
    gateway 192.168.0.1
    dns-nameserver 192.168.0.1
    wpa-conf /etc/wpa_supplicant/wpa_supplicant.conf

pre-up iw dev wlan0 set power_save off
post-down iw dev wlan0 set power_save on

allow-hotplug wlan1
iface wlan1 inet manual
    wpa-conf /etc/wpa_supplicant/wpa_supplicant.conf

When I lost connection I need to log in directly and run the commands in a root shell

# ifdown wlan0
# ifup wlan0

To make it work again. How can I avoid this?

The interface is not allowed for power save.

I had this problem a year ago. It was faulty hardware. The USB dongle. I switched to a different brand and issue disappeared.

Is this the on-board wifi ?. I run mine 24x7 as router/gateway, never noticed an issue, but I have a small UPS in front of it.

It is the onboard wifi. Because I want to have a fix adress I use a static configuration. Before I had an dhcp configuration. But I had this with dhcp too. So I tried static. Then I have googled to turnoff power save. But even with power save off I have the problem from time to time. I checked power save off with

iw wlan0 get power_save