This is a fairly common scenario when people are using Torrents. Some torrents are shared across 1,000s of users and pretty much over-saturate your poor router. Most routers are not built to handle large "Maximum Simultaneous Connections
", see this benchmark
to see some examples.
If I'm off target here and you are not using Torrents and/or have no clue what they are, then my apologies, and it would be a good assumption your router sucks and replace it with a newer faster one asap!