(In reply to Michel Dänzer from comment #6)

Tried with xf86-video-ati 18.1.0 :

Same delayed freeze.

I think the host gets overheated. The last line in kernel.log is 

Sep 19 20:44:44 hp2 kernel: [  337.131484] amdgpu: [powerplay] GPU over
temperature range detected on PCIe 0:0.0!

I was monitoring the temperature with 'sensors' command. Last output for amdgpu
sensor was :

amdgpu-pci-0100
Adapter: PCI adapter
vddgfx:       +0.82 V  
fan1:             N/A
temp1:        +190.0°C  (crit = +104000.0°C, hyst = -273.1°C)
power1:        1.04 kW (cap =  30.00 W)

Perhaps powerplay needs some fix ?

Regards.