If it was wireless, I'd say you aren't getting a good enough signal. When you tested your laptop, was it with the same cable? Because I'm thinking maybe the flat cable is too long and doesn't have enough shielding. There's a reason most cat 5/6 is round. They twist the wires in the cable, preventing crosstalk.
If you did test the laptop with the same cable, then check to make sure the cable isn't going close to any electric wires or anything. Well, do that anyway. Check for anything that could cause noise in the wire.
Solid tips, thank you. This really has become frustrating.