Why does my speed increases when i reboot my modem

I have a Belkin modem router and i usually have speeds around 6500Kps but i find that sometimes this can drop like today to below 1000Kps using the thinkbroadband speed test.
If i switch it off at the mains and reboot it the speed jumps back up to 6500Kps can someone please tell me why this is as it is becoming a bit of a pain to keep doing it.

many thanks for any replies.

i also have the same problem synk at anything between 6.5 and 8 meg and get download speeds of 5.5 to 6.5 ( when im not on a 512 fixed profile ???????)
but if my connection is inactive for a while I.E go out for a few hours ,at work etc synk is the same but actule speed can be down to 500 kbps ,reboot and all is well again very strange

I have to reboot my Netgear periodically, say every 10-14 days.

Just an aside, I now reboot daily.

I recently bought a set of remote controlled socket adaptors. Now, when I switch off my laptop at night I hit the remote 'all off' which kills the laptop transformer, print server, printer, router, PDA transformer, network drive and transformer. It kiils the lights in my study and saves a bucket of power.

The driver was the blue searchlight on the network drive