Found a solution to a problem that has been plaguing me for months.  My wifi would disconnect randomly, unable to recover and requiring a reboot.  The solution was not simple to find.  Sharing my notes here in case it helps someone else.

The problem seems to be the wireless interface’s Power Management.  And the solution was simply to turn it off.

I narrowed the problem down by watching the syslog until the moment it happened:

tail -f /var/log/syslog

In the logs I got messages such as:

upowerd[1147]: unhandled action 'unbind' on /sys/devices/pci0000:00/0000:00:14.0/usb3/3-2
kernel: [15922.365681] iwlwifi 0000:04:00.0: iwlwifi transaction failed, dumping registers
Timeout waiting for hardware access (CSR_GP_CNTRL 0xffffffff)
WARNING: CPU: 3 PID: 0 at /build/linux-hL8lyP/linux-4.15.0/drivers/net/wireless/intel/iwlwifi/pcie/trans.c:1973 iwl_trans_pcie_grab_nic_access+0xea/0xf0 [iwlwifi]
iwlwifi 0000:04:00.0: Queue 10 is active on fifo 2 and stuck for 10000 ms.

To disable Power Management, first run iwconfig to get your interface name:

iwconfig

You should see something similar:

<pre>
wlp4s0    IEEE 802.11  ESSID:"foobar"  
          Mode:Managed  Frequency:2.412 GHz  Access Point: 84:16:F9:7C:DC:13   
          Bit Rate=130 Mb/s   Tx-Power=22 dBm   
          Retry short limit:7   RTS thr:off   Fragment thr:off
          Power Management:on
          Link Quality=55/70  Signal level=-55 dBm  
          Rx invalid nwid:0  Rx invalid crypt:0  Rx invalid frag:0
          Tx excessive retries:2  Invalid misc:5182   Missed beacon:0
</pre>

In my case, mine is wlp4s0.  Yours might be wlan0 or something else.  As you can see,  Power Management: on

To turn it off (replaceing wlp4s0 with value):

sudo iwconfig wlp4s0 power off

This problem haunted me for months!  I even waited until Ubuntu 18 was released because I was certain it would resolve itself with a upgrade, but it did not.  I was one click away from buying a new motherboard.

Other helpful links:

https://askubuntu.com/questions/857103/random-trouble-with-iwlwifi

https://askubuntu.com/questions/85214/how-can-i-prevent-iwconfig-power-management-from-being-turned-on