Change to Thomson router - dramatic slowdown effect

[#1]

My father-in-law has been with Plusnet for a few years and I've always envied his connection speed. When he switched from BBYW to Value last year his estimated speed was 8mb/s (downstream attenuation 23), and that's what he got, with a solid IP profile of 7150.
Last month his (Plusnet-supplied) D-Link 2940B router stopped working. He requested a new Thomson router on a deferred contract, which was duly sent out, but I couldn't get up to his place to connect it until a couple of days ago, so he'd been offline for almost a month. I connected it in exactly the same place as before. It's a very simple setup - only one microfilter needed.
No problem with getting it going. The automated setup did its job and he was up and running within minutes. But when I looked at the stats I found that the sync speed was a lot lower than he'd previously been getting, at around 3800. This wasn't because of a high noise margin - downstream SNR was 6.5. I rebooted the following morning, but the sync speed came back at about the same level. His IP profile's now 3000.
I haven't heard that the Thomson routers are significantly worse at getting good sync speeds than other routers, so it probably isn't the router itself. There are new cables and a new microfilter, which came with the router. My best guess is that it's something to do with his wiring (I was surprised at how good his connection was when he first got it, given how ropey his phone wiring is). He's got a new fridge and a new Freeview set-top-box in the last month - I'm not sure if the demise of the old router, the old digibox and the (very) old fridge indicate problems with the mains supply.
Does anyone have any other ideas? Could it be that a line that's not used for a month is automatically reduced in speed? Sounds unlikely.
It's not really a problem for my father-in-law, as he's a very light user. But I'm just a bit puzzled about why there's such a big difference.
(I probably won't be able to act on any suggestions for a while, as I won't be up at his place for another couple of weeks.)
Graham

[#2]

have you tried it in the test socket of the master socket? (thats if his wirings not so old its not got a nte5 socket), try at least 2 diffrent filters and sets of cables from router to socket
if possable power down the items he got during the month down time and try a resync.
is his attention still being reported at 23db?
am sure after trying all that PN would send a router out to insure thats not at fault

[#3]

Thanks Asbo. He doesn't have an NTE5 socket, sadly. Next time I go up I'll switch in the old D-Link microfilter and see if that makes a difference.
I really don't think it's the router itself. I wondered if there were any settings that could be wrong, but I don't think that's the case either as I hadn't changed anything from the automatic configuration. As long as the router's actually providing a workable connection I'm not going to ask for a replacement.
Graham

[#4]

why not take your router up with you,if its your PN one it should log in fine with your account so no need to even change those settings which can rule the router out then

[#5]

Would that work? Wouldn't I have to at least change the username and password to match the one for my father-in-law's phone line? Good idea, though.
Graham