It's getting better the lower the resolution? That means something isn't outputting or inputting enough bandwidth, so it could definitely be a monitor, video card, (less likely mobo/PSU/BIOS). I'd RMA the monitor if I were you (you already had your video card replaced). Is it fine on any other PC (sorry if you already mentioned)? Just leave the PCI-E bus at 100 MHz. First thing I'd try is a new cable (again, sorry if you mentioned that already).