this EXACT same thing happened to my 990s just a month ago. Actually, i beleive the left and right channels were shorting together. I got a really far away funky distant sound.
First I reterminated them with a neutrik 1/8" jack which was a huge pain in the *****, partly because the jack was a tiny piece of crap (the only ones which parts connection had) and partly because each strand of the cables in the 990s are coated in a thin layer of plastic. This thin layer needs to be melted off, which was also a huge pain without flux. Anyway, I did it after much sweat, but I wasn't satisfied.
So i redid it with Canare starquad (in west village purple) with no techflex or anything. Markertek has it supercheap (¢39 a foot) and they ship almost immediately. They also have the Canare F-11 mini plugs which are much easier to solder. In case you have not seen it yet, I recommend you look at Jam Master J's DT 880 recable guide (search for Jam Master J - its in his profile).
The only suggestions i will add are that you can reuse the little brass clips that are in your headphones to pin the cable and prevent slipping inside the earpiece. You can also reuse the stock stress relief if you so desire (so that you don't have to drill). Zip ties are recommended, although I did not use any. Also, the number given for the right side cable length (16") is a bit conservative, I would recommend 20" just to be sure. You can always trim later.
This was my first DIY project (besides goofing off in physics lab or electronics lab), and I didn't find it to be incredibly difficult. Just have a nice clean, well-lit workspace. Oh, watch out for the super tiny, thin cables which go from the terminals on the drivers to the drivers. I avoided them easily, but I could see myself slipping and accidentally pulling one out which would certainly ruin the phones
