Only appears when the temperatures go normal(When it's hot, heated up, no issues)
Thermal expansion. Most materials expand when they are heated, when they expand that may cause a good or better connection on a cable, solder joint, or component.
When I worked on machines ATM'S and we could not identify the issue it became a matter of replacing suspected bad parts the thing is I always had a box of good know parts.......
I have no possible ideas where to look next it could be anywhere, it could be one component on the motherboard, a chip, capacitor...
I'll think more on this and let you know.
In the mean time.
Double check cables again, blow any dust away with canned air.
Re -seat any add on boards on motherboard...
Re- seat ram
You get the idea..