SBS Network slows down

This is an SBS 2008 server with about 15 client pcs. I

A few weeks ago a couple of users reported that that the network seemed very slow. I usually test this with a Utility called Lan_SpeedTest.exe it reports how quickly the server can be read from and written to. At that time Lan_SpeedTest gave me figures of about 30 Mbs write and 60 Mbs read (which is very slow). At the end of the day I rebooted  the server, and afterwards found that I was then getting figures 10 times faster ie 300 Mbs/600 Mbs. By the morning afterwards, the speed had roughly halved to 150 Mbs/300 Mbs, and a day later it had halved again.

Since then I've been monitoring the network speed and find that the speed is always about 300/600 after rebooting and usually it settles down to 75/150 after a couple of days. Usually it stays at this speed, but occasionally it slows further. Today is somewhat unusal. The server has been up for 10 days and I am getting 250/85 The write speed is not usually faster than the read. Looking at the server network performance graph, it is always close to 0% utilization. Can anyone suggest what is going on here?