Use
http://speedtester.bt.com/ to do the test.
Also how long is the cable between the router and the Master Socket? Minimize the length if possible.
Have you spoke with your ISP? Get them to do the KBD test, it can help them diagnose any faults on the line, or if they still use the older WOOSH tests get them to run those.
It sounds like when you're switching the router off and back on it's triggering your IP profile to change and after a few days it degrades (could be coincidence that it's over due to 'reset' when you restart the router, thus the speed increasing for awhile). Try the test above with everything else bar 1 PC/Laptop that you'll use, plugged in via network cable to the router.Screen shot the results or copy and paste them into a text file.
Kill ALL programs that use the internet (torrents, dropbox, SugarSync etc) and turn off all other devices that use the internet (other PC's laptops, xbox, PS3 etc) and do a BT line test from the only remaining PC/Laptop using a Network cable as Simon suggested. It could be the router or just the line length.
Remove the faceplate from the
BT master socket and plug the router into the BT style socket behind it, the master socket should have that line running horizontally across it about midway down the faceplate - it's how you identify the master socket from additional sockets that are 'tacked' onto master sockets cabling. Also do not use the micro-filter and unplug your phone too, use your phones cable to connect the router directly to the master socket's test socket.
Run one in the morning before work, one when you come home/before bed, record the results over a week, don't reboot the router in the mean time unless it's that bad nothing will load.
Unfortunately most ISP's will turn their backs on you and say well the service is only upto 8mb/20Mb, not guaranteed 8/20mb or blame it on the router being at fault(which it may be, but they can't tell without doing tests).
hth
