You've told us that you're using DD-WRT but you haven't mentioned which router and which version of DD-WRT. DD-WRT runs on many routers that perhaps aren't up to the task of delivering smooth streaming; especially if you turn on some of the QoS features to prioritize certain types of traffic.
Try plugging your computer directly into the modem and see what happens. If the problem persists, your ISP is probably at fault.
If, when you unplug the router from the modem, you hear a chorus of shrieks and wails, the problem may well be within your LAN as there's a lot more traffic than you know.