Bug#878221: linux-image-4.9.0-4-amd64: Screen flickers randomly with Radeon R9 270X since upgrade from 4.9.0-3 to 4.9.0-4

[besy]

I realized that the kernel I suggested as workaround
(linux-image-4.9.0-3-amd64) didn't completely solve the flickering
problem. However, it made the flickering occur very rarely.

I can confirm that the kernel "4.14.0-3-amd64 #1 SMP Debian 4.14.13-1
(2018-01-14)" from stretch-backports causes no
flickering at all with my Dell U2415.