Standard instructions are… standard, not for every case. Since you worry about it, I would expect you had gone and read manuals to check what the different options do. It’s still not late to do it 
. For example, we need info for the previous booted and failed session, not the current and successful one.
On Topic:
Unfortunately, there is no much difference that I can notice, but I have questions.
What is your configured display resolution, and how do you set it?
Is the LightDM resolution different from the user session’s (XFCE)?
[ 4.732] (II) Loading sub module "dri2"
[ 4.732] (II) LoadModule: "dri2"
[ 4.732] (II) Module "dri2" already built-in
[ 4.735] (==) intel(0): Display hotplug detection enabled
[ 4.736] (II) intel(0): [DRI2] Setup complete
[ 4.736] (II) intel(0): [DRI2] DRI driver: i965
[ 4.736] (II) intel(0): [DRI2] VDPAU driver: va_gl
[ 4.736] (II) intel(0): direct rendering: DRI2 DRI3 enabled
[ 4.736] (II) intel(0): hardware support for Present enabled
[ 4.932] (II) Initializing extension DRI2
[ 4.938] (II) intel(0): switch to mode 1920x1080@60.0 on VGA1 using pipe 0, position (0, 0), rotation normal, reflection none
[ 4.944] (II) intel(0): Setting screen physical size to 508 x 285
...
[ 15.897] (II) intel(0): resizing framebuffer to 1024x768
[ 15.897] (II) intel(0): switch to mode 1024x768@75.0 on VGA1 using pipe 0, position (0, 0), rotation normal, reflection none
Please, post :
pacman -Q xf86-video-intel
If that package is installed, you can try uninstalling it and testing for improvement.
When such behavior happens on a not-modified system configuration, it is a clear sign of one of two things AFAIK.
- Some hardware component is failing slowly (hardware)
- A race condition is affecting behavior (software)
So, my next question is "What have you changed on system or desktop (XFCE) configuration before this started happening? Think! Think!.. 
In case it is a user account issue, create a new user account and login to that, to test behavior.