Hi @krpwa
This is a puzzler. It is odd because normally if it was the GPU and the driver then the external display would be also be distorted. Is perhaps the replacement motherboard not the exact match for the old one? Have you looked at both to see if the part number is exactly the same?
Let me get this correct. The screen works fine with Windows generic video driver but goes wierd when the Intel driver is installed? What does Device Manager show for the Monitor before and after the Intel drive install? Your image looks like the frequency of the display is incorrect - but I'm only guessing.
Normally if I think it is just a driver corruption issue I suggest using the Display Driver Uninstaller app which completely removes all traces of video driver and settings.
What does the 3rd party reseller say?