Freebox and internet speed

Like others I find my internet with a Freebox slows down after it has been on a while. It has been swapped once when it was very slow and the chap said it was damaged.
I was wondering if I can swap the box for a decent Modem Router or does the Freebox do magic things that are required for connection.

If you have an ADSL connection then routers commonly negotiate line speed downwards until the connection is stable. Typically ours starts out at around 5mb/s & settles around 3.

Thanks Jonathan that explains a lot.