I'm just giving routerstats-lite a go now, polling ever 5 seconds, sharp jumps from 10 to 7 constantly. I'll post an image once its ran some time

I have a Billion 7402 coming tomorrow hopefully so It'll be interesting to see what stats that produces. as for the test socket not making any difference on the drop outs and snr fluctuations it comes as no surprise as I only have 2 phones on the line, and they're well filtered.

Let routerstats run allnight and day, there's no moment where it actually sits still, it jumps from 12db to 8db constantly, sometimes it goes down to 7.

There was 3 cases where it dropped stright down to 4db, but 3 times in a 24hr period doesnt bother me that much. The billion hasn't come so it looks like it might be next week before I get to try that out

Sounds like you're getting a lot of RFI or other electrical interference from somewhere. Have you got the router sitting on top of a PC or by the monitor?
Maybe the power brick for the router is failing?
Are all the phones filtered?
Have you got a skybox or something?

I might have missed it but I don't think you've said what router you are currently using?

Going by the firmware you mention (ZyNOS) it looks like it might be zyxel ?

It looks like these might use the TI AR7 chipset - this was dead loss (in a Netgear DG834v2) on my line with terrible SNR management.

I tried a few routers (see my RouterStats Graphs) and transformed things with a Thomson 585v6 (broadcom).

I think Billion use broadcom so it will be interesting to see if it can help your line

It is a Zyxel 660HWP-D1 which apparently is notorious for not handling SNR's below 6dB very well, probably based on the chipset. I just checked the router gubbins and sure as damn it, its running the TI AR7 07.02.03.00 chipset.

My Routerstats graph is very similar to the graph for the Netgear, very spikey and between 12 and 7db.

The Billion 7402R2 uses a Conexant chipset as far as I'm aware, but does come with features to tweak the SNR like other broadcoms. I'm going to try this Billions since I picked it up for 15 quid, so even if it doesnt make it any better, I'm not at much loss

I'm a radio amateur so I'm very RFI concious, I've made sure the extension to the office doesn't have many staight long runs and clipped on some RF Chokes as well.

The only thing in the house that could possibly cause the problems is the dimmer switch in the adjacent room, but with it on or off seems to make no impact on the SNR.

As I mentioned previous (I think) all the phones are on ADSLnation filtered faceplates

So what frequency is the noise coming in on?

Argon 330 (7402), Argon 432 (7402R2)
The original 7402 was produced with a GlobespanVirata chipset. Later production incorporated the Argon 330 chipset, a GlobespanVirata legacy product from Conexant. The 7402R2 incorporates the newer Argon 432 chipset, bringing it into line with the rest of the 74xx family.

The above can be found here, about two thirds of the way down the page

That's anyones guess, could be interference coming in on the line before it even gets into the house, you never know!

I've not yet ruled out that the noise could be on the extension line that comes up the house and through the loft although there is the chokes on it, I'll have to do a run with router stats with it in the master socket and see if theres any difference.

If its that, its not too hard to just replace the poxy phone wire with STP or FTP cable.

Just did a run with routerstats plugged in at the master socket, no difference what so ever.