enabling identifying network cable unplugged

So here's a fresh spin (and not in a good way) on the seemingly common port-flapping issue, where a windows vista/7 machine's ethernet port vacillates between "enabling, identifying, network cable unplugged."

I've tried the fixes I've seen around, like forcing the speed to 10 or 100Mb duplex,etc. But I think the problem goes deeper than that -or drivers or faulty NICs. Its acting like it's in the cabling. . . but not. Any advice would be welcome.

Just switched from one DSL provider (Bell) to another (Teksavvy). Connected via a dry loop. When the Bell tech first set it up a year ago, he did it like this: Patch cord frome the Bell 2Wire modem/router combo to a female coupler. Female coupler attached to cat5 cable running about 25/30 feet along baseboards around the corner to another female coupler. Patch cord from female coupler to NIC in the back of my box. This worked fine.
New set up with Tek savvy is as follows: DSL modem via patch cord to (wireless) D-Link router. D-link router connected to computer by same aforementioned setup. This results in the port-flapping named in the title of this message, on the hard line only. Any change in my computer was ruled out by connecting a brand new, never been connected laptop and the same terminal as my computer. Results in port flapping. The same laptop connected to the router by just the patch cord results in a perfect connection. I've tried moving the patch cords around, it doesn't change the results, so they're fine. I have no way of testing this long stretch of installed cable in the middle, with the couplers on it, since I don't have an equal length cable to try out anywhere, but I'd be surprised if that just died after a year's use exactly when I connected new hardware to the other end of it. Running windows 7 with a RealTek NIC on my box. D-Link router (will check the model later, at work at the moment), and TP-Link DSL modem. The system delivers wireless internet flawlessly. Could this be interference? I don't think the cable is shielded, but the only source of RF added to the system is the D-Link Wifi. . . baffled. Suggestions welcome. Sorry for the essay.

-Marcus