@Jules_F Somehow I managed to downgrade 2 out of 6 M5 decos with 1.9.1 installed. The other 4 are not downgrading. I used Tftpd as explained in https://www.tp-link.com/us/support/faq/2958/ I don't know why that is... Running only the 2 downgraded devices, made my network 100% again. Still trying the other 4. Later if I not succeed, will try to connect the rest to my network and see what happens. Shame on tplink for ruining the devices. 3 years running perfect 'till I decided to upgrade the firmware. I'll never do that again that's for sure.

  @David-TP I fully agree, i have about x14 Decos M5 Spread around a big place and all was fine on 1.7.8. 
Since I upgraded to 1.9.1, both my systems have gone haywire.
My Decos are making noises, heating up, losing network...  
Their should be an easier way to revert back. 

When i visit the page i can only revert back to 1.8.5, is this as stable a the 1.7.8?

Thanks!