DisplayPort and Intel HD cause GPU hangs

My system has problems with the displayport connection. This is indicated by several problems that, at the first glance, do not have anything in common. The reason why I claim DP for being the cause, is that when I connect another monitor via DVI these problems just vanish. When I put monitor into sleep it won't wake up. Journal contains: [drm:intel_dp_start_link_train] *ERROR* failed to enable link and sometimes [drm:i915_hangcheck_elapsed] *ERROR* Hangcheck timer elapsed... GPU hung Qt applications need a few seconds to start. And meanwhile freeze X. Today I had a complete never-ending system freeze. As a followup KDE start is painfully slow and accompanied by multiple freezes. I use an up-to-date Arch System on an i5-4590, using the Intel HD4600. Here is dmesg with drm.debug=0xe comandline. I cut about a million "[drm:drm_dp_i2c_do_msg] native defer" lines to make it cleaner. Intel drivers are installed. The config: # for i in /sys/module/i915/parameters/*; do echo $i=$(cat $i); done /sys/module/i915/parameters/disable_display=N /sys/module/i915/parameters/disable_power_well=1 /sys/module/i915/parameters/enable_cmd_parser=0 /sys/module/i915/parameters/enable_fbc=-1 /sys/module/i915/parameters/enable_hangcheck=Y /sys/module/i915/parameters/enable_ips=1 /sys/module/i915/parameters/enable_ppgtt=1 /sys/module/i915/parameters/enable_psr=0 /sys/module/i915/parameters/enable_rc6=-1 /sys/module/i915/parameters/fastboot=N /sys/module/i915/parameters/invert_brightness=0 /sys/module/i915/parameters/lvds_channel_mode=0 /sys/module/i915/parameters/lvds_downclock=0 /sys/module/i915/parameters/lvds_use_ssc=-1 /sys/module/i915/parameters/modeset=-1 /sys/module/i915/parameters/panel_ignore_lid=1 /sys/module/i915/parameters/powersave=1 /sys/module/i915/parameters/prefault_disable=N /sys/module/i915/parameters/preliminary_hw_support=0 /sys/module/i915/parameters/reset=Y /sys/module/i915/parameters/semaphores=-1 /sys/module/i915/parameters/vbt_sdvo_panel_type=-1