Repeated sync loss with different routers

[#1]

I probably have 2 different problems here. Hopefully something in what I report will trigger some idea with somebody who knows more about ADSL (Max) than I do (which is most people).
It started when I tried to set up a D-Link DSL-G604T for my in-laws. This initially worked fine, but after a period of maybe 20 minutes it started to go in and out of sync (repeatedly within the space of seconds). OK I thought, they probably have a poor line quality (they are after all in a fairly rural area). But when I tried it at home I got exactly the same result. The thing is the DG814 which I normally use has never had any problem.
Looking at the problematic G604T (on my own line) this morning I have:
SNR margin (down):  7 dB
SNR margin (up):      26 dB
Attenuation (down): 34 dB
Attenuation (up):      16dB
I also found that I ended up with much poorer speeds with the G604T (in the order of 1Mbps compared to 5Mbps with my normal DG814).
Is the G604T just particular bad at dealing with low SNR? Unfortunately the DG814 doesn't give me any SNR figures so I can't compare.
I have also got a Cable & Wireless 802.11G Wireless LAN ADSL Router. The mystery deepens with this as I have figures  of:
SNR margin (down): 12.5dB
SNR margin (up):    23dB
Attenuation (down): 20dB
Attenuation (up):    16dB
... and yet I have what I suspect is the same problem as I am experiencing with the G604T. It works OK for a while and then begins to connect and disconnect (the logs are fairly poor so I find it hard to say for certain that it is doing the exact same thing as the D-Link). It actually gives results comparable to the DG814 on speed test so is better than the D-Link in that respect).
So I suppose I was just wondering if these symptoms ring a bell with anyone.  e.g... is my line just poor (as wellas my in-laws)? Is it an SNR problem? Have I consistently misconfigured the routers other than the DG814?

[#2]

Hi there,
You haven't made any reference towards your filtering or internal wiring, which is often more important that trying alternate hardware.
Are all of your sockets filtered?
Have you tried changing filters?
Have you tried testing from the test socket behind the master faceplate?
Let us know

[#3]

I have one phone and one router both connected a short distance away from the single filter I am using from the only socket.
I have another filter I can try. I will check this out and also try the test socket when I get a chance.