I think I remember running into issues like this in the past. The internal clock for the integrated graphics may be going out and when in lower power states may start to cause issues... Sometimes it presents as terrible artifact on the internal monitor as well.[br]
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If you havn't gotten any further in this solution but have noticed that when an external monitor is plugged in - everything is fine, I would recommend picking up a 4k or such HDMI dummy dongle (those things that video card crypto miners used to get the most out of their cards) to force a higher power of the graphics to be used.[br]
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Also, if you havnt yet, give memtest a try and verify that your ram is working well.