I'm back to 14 days uptime.

On day 112 I noticed the built-in browser (grob) and email client (fenix) could no longer access the network, although Wazapp, QmlMozBrowser had no problems. Also the 'Connect to internet' dialog which gives a choice of networks would not appear.
I found a temporary solution in this thread, which was to run:
This would allow the built-in browser and email to work, however this had to be repeated after several hours.
The 'Connect to internet' dialog still did not work.

On day 113 I was using navigation in Nokia Drive and lost GPS signal due to a rain storm. The GPS icon continued to flash next to the time display for hours after I had closed Nokia Drive. I couldn't use GPS functionality in any other application.
I was able to solve this by killing /usr/bin/positioningd.

On day 115 my N9 switched off while sounding my wake up alarm. I think I didn't plug it in to charge the previous day.

After allowing it to fully charge, I switched it on again and the problem with the 'Connect to internet' dialog, the built-in browser and email had gone.