I wonder what happens next ?

[#1]

My connection has been completely stable for abt. a month with bandwidth at about 1.25 Mbps, S/N margin around 17 Db, attenuation around 67  Db and with very few errors.
Today we had a power cut of around 30 seconds and since it came back on the bandwidth  has been 2563 kbps, s/n margin 6.o, and about one million FEC errors per hour!
I wonder what the clever system will do now? ( this is a rather rhetorical question as  I've a fair idea of the answer, but any kind or helpful thoughts would be welcome  )

[#2]

You could buy a modem that allows you to override the target SNR, so you control the balance of speed against errors.
You could buy a UPS for your modem so it doesn't happen again.

[#3]

With a page refresh (of this thread) and virtually nothing else, I racked up 14000+ FEC errors in 2 minutes - about half your rate - and have had a stable sync for 190.5 hours (3207kbps/60.9dB attn).
I get "hesitations" sometimes with page loads, with a number of "netMakeChannDial: err=-3001 ..." errors logged, which seem to be PPPoA errors/drops based on information from searches.
As I get a relatively low increase in CRC errors (5 in the last 19 mins), FEC seems to be doing it's job keeping the connection running

[#4]

Thanks for replies.
@ PeeGee.  Interesting !
@ Purleigh. Two nice suggestions, but a little techy for me and, especially, difficult to justify for my rather basic computer use. (I have enough trouble now with SWMBO  thinking that time spent "playing with the computer" could be put to better use - and she could be right)

[#5]

From memory of my ADSL days some routers reported vast numbers of FEC errors but everything still worked just fine. The ones I'm talking about were Speedtouch/Thomson and I think the PN issued ones are newer verions of those.
It looks to me like you had a long period of stability and the DLM reduced your target margin probably from 15dB as it gets set in 3dB steps back down to the default 6dB but with no resync you didn't see that. So long as you don't get large numbers of CRC errors or a forced resync you could well continue at the higher speed. Have you optimised your internal wiring at all?

[#6]

The internal wiring is unchanged.  So far the d/l speed is unchanged at +/- 1.03 Mb/s so I'm waiting to see whether it improves in the near future - I feel that messing about with the router connections during the holiday season might be a bad idea as in the past the d/l speed has dropped to near zero and PN had to be asked to do a reset, which I guess might be tricky around now. 

[#7]

It may worth a quick look at Forward Error Correction for why large numbers of FEC errors are not worth worrying about whilst the CRC error count is low(ish).
The latter is a prime mover in the DLM algorithms as CRC errors require retransmission of the data. Now and again, I get a burst of CRC errors that upset the DLM and my line gets "adjusted" (often a number of resyncs in a short time), but it then settles and works its way back to between 3Mbps and 3.6Mbps with a SNRM around 6dB or so.

[#8]

Just an update for info.  During Sunday night the bandwidth dropped to abt. 1850 kbps and d/l speed abt 1500 kbps -quite good for this location.
Then during last night there was another short power cut and the bandwidth is now 1166 and d/l speed abt 1003 kbps which seems to be about par for the course, unless someone can be persuaded to keep a constant eye on it and adjust as needed. I should be so lucky!