Created attachment 96597 [details]
Xorg.0.log for failed start of X server

@Thierry: understood, I still do not experience any hang but I'll keep eyes
open. Meanwhile it happened again that X server failed to start and this time I
have saved the X log. It seems that the problem is:

[     2.501] (II) [KMS] drm report modesetting isn't supported.

while for successful boots I get:

[     4.203] (II) [KMS] Kernel modesetting enabled.

Also, for successful boot I sse in the log:

[     2.510] (II) xfree86: Adding drm device (/dev/dri/card0)

which is missing in the log for failed X start.