wow, I wasn't notified about all the replies here since I posted in May 2019. Unfortunately the problem has now occured 3 times since April. I have gone through all the dist-upgrades but the issue still persists.
This is soooo annoying as I run my home automation system on the Orange Pi. The WAF is going dramatically down :-(
Will try the CPU speed setting as well, but usually it takes a few months for the issue to re-appear on my side