So I was out on a ride tonight and starting to go downhill it felt like I had a loose tire. The bike was noticeably shaking from side to side. I stopped, checked the quick releases and brakes and such on both tires and it all seemed fine. Got back on, still an issue. Once I got above a certain speed the feeling seemed to dissipate (or it could just the it was masked by the normal bumpiness of the roads here at speed). However once I drop back down its back. I'd say between about 10-20 mph range the bike is shaking very noticeably. IE on a downhill stretch of paved road if I take the weight off the handle bars it is shaking from side to side. But the feeling is almost like the whole frame is twisting, when I looked down it wasn't just the handle bars but the entire frame seemed to be rocking side to side.
My back tire is slightly bent, but it has been that way for a while. It's a by product of riding a fairly narrow tire on the pot holed and dirt roads (more like wide MT bike trails) here. I was riding a particularly bumpy section pretty aggresively, but the tire doesn't appear to be any more bent than it was when I started. Any ideas for a cause?