That to me looks like a video problem, most likely cable or VRAM. You'll need to perform some backward process of elimination.
Try swapping out the cable, connectors and even the monitor one-by-one, just to eliminate that side of the hardware. After that, try using the standard motherboard video connection (if it has one) and turn off the external video card in the BIOS.. If you don't have a motherboard video connection, then try swapping out the video card if you can get your hands one. If either works, then the culprit id the video card. If it still plays up, then you'll need to start testing your system RAM.