If it only occurs when warmed up, that means it is more likely a hardware problem. If you are running the Dell diagnostic that is starting only after a reboot (not starting up as a Windows program) then you are running software that is totally distinct from the Windows install which would indicate that Windows software is not the issue. If it is a hardware problem, it could be the graphics chip (part of the motherboard) or the display circuitry (part of the display card). I'm not sure there is a good way to tell the difference. One thing you can try is to run an external monitor and see if the ever happens. If it does, that would point to the motherboard.