How can I test the GPU/motherboard?

I have tried replacing the LCD LVDS cable and reinforced the connection to the display for good measure by using more tape then I actually need , as can be seen here:

At this point, I know both of the LVDS cables are probably okay so I am thinking the issue with this notebook is now related to the GPU/motherboard or panel. If I don’t use the notebook for a while, the LCD glitches. It doesn’t always do it and it always seems to recover, so I’m thinking there’s a display or GPU issue. While I have a parts unit to get a panel from with the correct resolution (XGA cables only work with an XGA display), I can use that to further troubleshoot it. However, while it’s not a hard repair it is a time consuming repair. I need to pull it from that unit, install it onto the one I’m trying to troubleshoot and protect the old part in case of a bad diagnosis. I’d like to be 100% sure before I open that unit and pull the LCD.

Is there any tests I can do that will rule out a GPU problem? I don’t ever see it on an external display so I’m fairly confident the screen is the problem at this point, but I’d rather test the GPU to be sure. Diagnostics for Windows 4.12A is rudimentary and only detects major GPU problems and passed it.