Line dropping after switch to Unlimited -- new training period?

[#1]

I recently switched to the Unlimited plan. Since then, my connection has been dropping out more and more often -- at least twice just today. For the last year or so it had been syncing at 3dB and about 18Mbps and staying up for weeks at a time (albeit with silly numbers of errors); now the speed is creeping down and the SNR is up to 4.5. Is it just a coincidence, perhaps connected to the wet weather, or is it possible that the plan change has triggered a new "training period"? Or is my three-year-old Thomson modem on the way out?
It's hardly at the level of a fault, but it is a bit tiresome and it would be nice to know what's going on. Trying it on the master socket is not something I can do lightly -- it would involve buying or borrowing an extension cable to run power either through or across at least one doorway -- so I don't want to do that except as a last resort.

[#2]

@saguaro
What product were you on previously? Changing from Essentials or Extra to Unlimited should have no effect on your connection to the exchange. The router would not even need to re-sync, just the PPP (Internet) session would drop (once) for a few seconds to allow the changed traffic management profile to take effect. A new training session is not needed (the DLM will not know anything has changed).
The problem you are seeing is most likely weather related, though could be a router problem  Reporting a fault at http://faults.plus.net and working through the tests is suggested. unfortunately these do involve connecting to the test socket.

[#3]

Thanks. That's one possibility ruled out, then. I don't actually have a test socket, just an old-fashioned (T logo) master socket, very awkwardly positioned. I think I'll give it  a few more days before I start messing with it.
(Edit to answer the question: I was on Extra.)

[#4]

Can you try going into the routers GUI and choose disconnect / reconnect, you wont lost sync and it should not to trigger the DLM

[#5]

As it's losing sync without my intervention, I don't see how that would help, but thanks for the suggestion.

[#6]

The lack of a test socket does make diagnostics more difficult. The best that can be done is to unplug everything except the router and connect that into the master socket.
Does a quiet line phone test (17070 Option 2) (using a corded phone if possible) show any noise around the time sync is lost?

[#7]

Quote from: spraxyt

Does a quiet line phone test (17070 Option 2) (using a corded phone if possible) show any noise around the time sync is lost?

Embarrassingly enough, the only corded phone around here is so ancient that it only does pulse dialling, so no options. And plugging it in to see if it worked at all just resulted in knocking out the connection again.  It had been stable for 18 hours up to that point,but then it hadn't been raining. I suspect my best course for now is just to leave well enough alone.
dick:green Quote fixed.

[#8]

Doing a quiet line check with a corded phone is best, but if a suitable one isn't available test using a cordless.

[#9]

Hi, it's definitely still dropping: <img src="http://community.plus.net/visualradius/generated/image13887469069486.png"/>
Well worth raising it to us at http://faults.plus.net so we can get it investigated but please do the quiet line test before hand so we know whether it's a phone line fault or if it's the broadband so we know which one to raise the fault on

[#10]

I'm getting increasingly suspicious of the modem. It just froze solid on me for the second time since last night -- power, broadband and internet lights on solid and no wifi, this time -- requiring a power-cycle. Quiet line test immediately after I got back on line was fine, but I'll try to check it again when it's actually out -- now that I have the number written down on paper!
(The modem first started occasionally acting up around the time I put Linux on my netbook, and yesterday I introduced a new Win8 laptop to the network, so it's possible there's something about one or other of those that keeps crashing the wifi, I suppose ....)