Over the past 3 years, the biggest obstacle to my broadband experience has been the fluctuating/low-spiking SNRM on any of many routers I've tried. Usually fairly consistent within +/-1db, every so often, a large downspike or temporary drop of up to 4db wrecks my tuned SNRM (tweaked to 7 to get best sync rate), sometimes causes disconnections and has contributed to my high default SNRM (15).
My line is long, has an attenuation of 68.5db, but at brief good times, eg last January, enabled me to hold a sync of 2016 for 4 days at an SNRM of 6-7db. Normally, 1440 to 1728 kbps is my current target, at SNRM 9'ish, and I can usually keep a profile of 1250 (even 1500). At good times, my routers can hold on as low as 2.5 to 3db, but more often they don't like going below SNRM 5.....so I now allow for a 4db spike, and tune my routers to SNRM 9......a fairly successful procedure. Lowering the SNRM to 7 and below puts me into 'dangerous' territory.
On 2 occasions, I've identified the cause; once it was my neighbour's problematic 4-way power strip...its neon flicked unusually, and it made a buzzing noise. I replaced the strip for him and that cause ceased. The second time was when the same neighbour fitted a replacement PSU to his PC (I helped him). After the PSU was in place, whenever his PC was on, I lost 2db of SNRM. His house is 50m away, but we share the same telegraph pole for BT line; he moved house a couple of weeks ago, so that problem has also ceased. But there are other fairly regular drops, of predictable pattern, which I can't identify, and there are mysterious very short-duration downspikes of 0.25 to 4db which don't have any pattern I can spot.
Because I believed the interference to be mains-borne (our vacuum cleaner also causes similar spikes), and trying off-the-shelf mains conditioners seemed to have no impact whatsoever, I'm now in the process of testing my theory by totally isolating my router from the mains supply, and powering it from a car battery via an inverter. The inverter connects to the battery, the router PSU plugs into the inverter output of 240v and thats it - no link from the router to the mains. I've determined that I can charge the battery without interrupting router operation, but of course charging gives a route to the mains, so I've yet to investigate whether thats a problem.
After 3 hours of operating, I'm impressed with the stability of the router (DG834 V4) SNRM - not even 1 downspike recorded, and thats unusual, and the variation is only +/- 0.1db. I don't yet know how long the (old and 16AH) battery will power the inverter (its currently (!) drawing 1A at 12V, so might last for 12+ hours), but if the experiments conclude satisfactorily, I'll invest in a 55AH battery that matches the one in my car, and I could put the charger onto a pre-set timer during the quiet nighttime. I'm waiting for the next predictable time for SNRM drop of (previously) 2db - tomorrow morning between 6.30 and 9.00 - to see if that is prevented.
Total cost so far: £16 for power inverter; battery was free from recycling heap at local garage.
You may wonder if the DG834 could be powered directly from the battery - its PSU says 12v at 1A, so maybe it could.....but the route I've chosen seemed the safest to me - it ought to give a more stable voltage to the router. And of course the direct battery route would not be immediately suitable for non-12v-DC routers, whereas this inverter method can be used for any mains-powered-PSU router. Why not use a UPS? - well most UPS are 'off line'; during normal operation the mains is fed to the output, and only when mains power is off does the battery drive the output. 'On line' UPS, which can provide galvanic isolation from mains at all times, are very expensive indeed, but also offer a solution.
I'll update this posting once I get more results in.