Keep getting new WAN IP address, and DSL line is interrupted

[#1]

Basic question: is it worth going through the fault procedure for this:
I'm on FTTC, 40/20 package. Several times a day recently I'm getting a new WAN IP address, which probably means a previous PPP disconnect although my router doesn't explicitly say so (I'm currently using a ZyXEL NBG4604 that Plusnet gave me when I was on the 80/20 trial a while back). In addition, and not always at the same time, my DSL connection gets interrupted briefly - i.e. the DSL light on the Openreach modem goes out, then flashes, then goes back to steady, the whole process taking under a minute.
Here's what the router is logging when I get the new WAN IP address (the DHCP ACKs for LAN addresses are just context). Read from the bottom: the process begins with "PPP, CHAP Authentication Succeeded"
1 Aug 18 19:17:08   DHCP Server, Sending ACK of 192.168.n.n
2 Aug 18 19:17:06   DHCP Server, Sending ACK of 192.168.n.n
3 Aug 18 19:17:06   DHCP Server, Sending OFFER of 192.168.n.n
4 Aug 18 19:08:29   NTP, Local time=2014/08/18 19:08:29
5 Aug 18 19:08:29   NTP, Daylight saving from Month/Day to Month/Day: 3/29 ~ 10/25
6 Aug 18 19:08:29   NTP, Daylight saving status: Enable
7 ndon   NTP, Time zone = (GMT) Greenwich Mean Time : Dublin, Edinburgh, Lisbon, London
8 Aug 18 19:08:28   NTP, start NTP Client
9 Aug 18 19:08:25   WAN, start TR-Agent
10 Aug 18 19:08:24   UPnP, Start
11 Aug 18 19:08:22   UPnP, Stopping
12 Aug 18 19:08:22   Reloading, Reloading SNMP server
13 Aug 18 19:08:21   DNS, start DNS Proxy
14 Aug 18 19:08:20   QoS, Stopping
15 Aug 18 19:08:20   NET, start IPv6 Firewall
16 Aug 18 19:08:20   NET, stop IPv6 Firewall
17 Aug 18 19:08:19   NET, start Firewall
18 Aug 18 19:08:19   NET, start NAT
19 Aug 18 19:08:19   NET, stop Firewall
20 Aug 18 19:08:19   NET, stop NAT
21 Aug 18 19:08:19   WAN, IP changed, restart services
22 Aug 18 19:08:19   WAN, New IP = 31.185.n.n
23 Aug 18 19:08:19   PPP, CHAP Authentication Succeeded
24 Aug 18 19:00:02   DHCP Server, Sending ACK of 192.168.n.n
25 Aug 18 18:59:38   DHCP Server, Sending ACK of 192.168.n.n
It's a pain while the services are stopped, as I then can't connect to the net - which bizarrely seems to affect the wired PC for much longer than the wireless devices I've got.
It's quite possible that there are several different things going on at once, of course. I moved to the ZyXEL to test my local equipment before raising a fault, but then the DSL problems seemed to settle down so I didn't bother going through the faults procedure as I'd done it once last April. I may now try going beck to the Netgear WNR1000v3 I was using before (also Plusnet-supplied, before they started giving everyone the Technicolors), as its logs do tell me when there's a PPP disconnect
I should add that the telephone line is intermittently crackly, and that I've seen the DSL line go down just after I've picked up the phone to answer a call.
I did report this kind of line fault last April when the DSL problems were worse then they are now.. Firstly a phone engineer tested the line and said it was OK (classic intermittent fault behaviour). Then a proper broadband engineer did a 'lift & shift' at the cabinet and changed the (probably faultless) Openreach modem for good measure. So I don't know whether to start the whole rigmarole again. Should I wait until my Plusnet 'connect speed (IP profile) drops below 40 as it did in April, or should I start a procedure now before it gets worse (which, of course, it may not).
Sorry - too many things at once here, and they're all nuisances rather than showstoppers.
Graham

[#2]

Hi Graham,
Sorry to hear that you've been seeing problems with the connectivity dropping intermittently.
When I started reading I thought this was possibly going to be a hardware issue (which there could still be) however as I read on I thought it best to check both the phone line and broadband with a quick fault diagnostics which has turned up a possible problem on the broadband side of the service. Given this I have now raised it to the suppler for you and we're now awaiting further details back from them pending further tests and checks.
You'll be able to follow the progression of the fault on the support ticket I've raised on your account and the next update should be applied when we receive the details of the testing back later this week.
On a side note, it may well be worth testing the alternate hardware if you have it to hand, just so we can eliminate this as a possible cause/contributing factor.

[#3]

Thanks very much for following that up, Adam. I saw that the question has been raised. I won't reply on the ticket itself but I will follow the progress of the query you've raised with BT.
I'll change the router back to the Netgear later this morning, but I'm pretty sure it's not a router problem. My best guess is that there's something not quite right with the line somewhere between my master socket and the fibre cabinet. It may be hard to detect. The phone line can start off clear and then 5 mins into a call become so crackly I have to ask people to repeat themselves.
Do you want me to give you a list of times when my WAN IP changes? Or would that not add anything to what you already know?

[#4]

Hi grahamt,
It's certainly not a problem!
If you want to make a mental note of the times you see the WAN IP change that would be best for now, we may not need to know but it would be helpful if you had them to hand just in case we do.

[#5]

OK - yes, I'm keeping notes, though that's less important now I've switched back to the Netgear, which sends me a weekly email of its log. Since I swapped routers at lunchtime there has been one disconnect (16:20:05).