Internet Speed dropping while watching Netflix.

Hello, I was hoping someone could help me diagnose an issue I'm having with Comcast and Netflix. On a good day when I'm not doing anything major on the internet I can run a speed test and get an average of 45mbps down. But if I start playing a netflix video on my xbox360, and I then run another speed test my speed drops to just under 10mbps. There is no way Netflix is sucking up 35mbps. Is Comcast throttling my speed when I try to watch Netflix? Is there something else going on? Is there any other tests I can do to figure out what's going on? I've called Comcast to explain, and each time I'm told it's likely the router and that I can bring it in and swap it out, but will that really help?