It's a guess, but I think that the phrase " Warm Up " may be relevant it sounds like somewhere there's a bad contact that is fixed for want of a better word when the metal heats up and expands. Pinning it down may be difficult, check that all cables are plugged in firmly and where applicable screwed in place. Then assuming the output is from a graphics card and not the MOBO make sure it's seated in the slot properly. If the graphics is from an onboard chip on the MOBO I'm not sure what to do to fix it, other than suggest fitting a dedicated graphics card.
OnePlus 5 review