Does anyone here know how a faulty extension cord could slow down an internet connection?

My problem: I have an extension cord, which, when used to bring power to my modem and router, which are located in the laundry room at the phone demarcation point, slows my connection to a crawl. On DSL I'm normally getting around 1.7 Mbps. With this stupid orange extension cord I might manage 50k if I'm lucky. I know it's not grounded properly, but I don't see what difference that would make as the ground wire does not normally carry a charge that could create an EM field. When I use a different extension cord, I'm back up to my normal speed again. I'm thinking it might be the shielding, maybe because the apparently faulty one is of a thicker guage, or maybe the longer cord has more length to create an EM field? I'm just guessing here though... This phenomenon has me baffled; I've never seen anything like it. Although I can solve the problem with a different cord, it's kinda too short, and I'm gonna be racking my brain with it until I find an answer. Has anyone seen this problem before?