Sync speed slowing down

[#1]

Since I switched to max my ADSL line was giving a rock steady 4.5Mbps / 445Kbps connection.  Over the last month or so the modem connections speeds have been slowly dropping and becoming erratic.  It has gotten so bad that I took the time to set up a monitor to track the downstream speeds.  I moved the router directly onto the master socket about 10 days ago with absolutely no improvement. 
Today I saw an all time low.  The router connection was dropped up stream and it resynced at under 3Mbps.  So that is a 1.5Mps drop for no apparent reason.
I rely on my ADSL line maintaining a half decent speed to work.  It appears the speed I can achieve is falling just when my customer's expectations are rising
What the heck is going on and is there anything I can do about it.
[tt] 
Max        Average  Current
4192 kb/s  3976 kb/s  3296 kb/s 
3904 kb/s  3682 kb/s  3104 kb/s [/tt]
The upstream speed is always 445Kbps.  Throughput is always about right for the speeds the modem has connected at.

Test1 comprises of Best Effort Test:  -provides background information.
    IP profile for your line is - 3000 kbps
    DSL connection rate: 448 kbps(UP-STREAM)  3328 kbps(DOWN-STREAM)
    Actual IP throughput achieved during the test was - 2138 kbps

[#2]

Hi msssltd,
sorry to hear that you've got problems, there could be a number of reasons. Are you able to access your router stats? That may help if the problem isn't a simple one.
Carry out some basic checks first. Have you changed anything at your end - ie. changed phones, filters, added any extensions or anything else? Is there any crackling noises on the phone line?

[#3]

Hiya,
I can see that you've been trying to test the connection from your master socket - thanks for that.  Can I ask if you have also tried using either a different filter and or modem/router?  It might also help if you can plug your filter and router directly into the test socket as that might help drive up your sync speed a bit too.
Let us know how you get on.

[#4]

Nothing has change inside the premises (particularly).  The voice phone is fine.
There have been a number of routers on the line ranging from the cheapest Zyxcel's to pretty expensive Cisco's.  The sync speed's vary a little across makes and models but they are all syncing erratically.  The router producing the graphs is a brand new Vigor 2820n.  A reboot yesterday evening got it up a couple 100K from the 12 o-clock low.  My opo rebooted it this morning and it's back up to 3.8Mbps
We are stretched for time at the moment and I could do without the messing around, but I will see if I can get to playing with the filter this evening.

[#5]

Best time to re-boot the router is usually morning, when the SNR margin is usually higher. If you can take a look at your router stats and see how it's fluctuating. Don't re-boot the router if the SNRM is lower than when you last rebooted, otherwise your sync speed will drop.