The problem is that my ADSL modem (Draytek Vigor 120) will often disconnect, and will take a long time to re-establish WAN connectivity (usually quite a few hours). Restarting the modem does not fix this, and all I can do is simply wait. Here is a sample from the modem's log (as seen via my router's admin UI; I can't seem to find the corresponding log in the modem itself)
Dec 7 06:01:09 pppd[459]: Serial link appears to be disconnected.
Dec 7 06:01:11 WAN Connection: Fail to connect with some issues.
Dec 7 06:01:15 pppd[459]: Connection terminated.
Dec 7 06:01:15 pppd[459]: Modem hangup
Dec 7 06:01:26 pppd[459]: Connected to 00:50:7f:f2:e5:fe via interface eth0
Dec 7 06:01:26 pppd[459]: Connect: ppp0 <--> eth0
Dec 7 06:01:56 pppd[459]: LCP: timeout sending Config-Requests
Dec 7 06:01:56 pppd[459]: Connection terminated.
Dec 7 06:01:56 pppd[459]: Modem hangup
Dec 7 06:02:06 pppd[459]: Connected to 00:50:7f:f2:e5:fe via interface eth0
Dec 7 06:02:06 pppd[459]: Connect: ppp0 <--> eth0
Dec 7 06:02:36 pppd[459]: LCP: timeout sending Config-Requests
Dec 7 06:02:36 pppd[459]: Connection terminated.
Dec 7 06:02:36 pppd[459]: Modem hangup
Dec 7 06:02:46 pppd[459]: Connected to 00:50:7f:f2:e5:fe via interface eth0
Dec 7 06:02:46 pppd[459]: Connect: ppp0 <--> eth0
Dec 7 06:03:16 pppd[459]: LCP: timeout sending Config-Requests
Dec 7 06:03:16 pppd[459]: Connection terminated.
Dec 7 06:03:16 pppd[459]: Modem hangup
The problem has changed a little over the last week. When it first occurred, we would experience significant degradation in performance when the connection restored. We usually get around 10-13Mb/s, but this would degrade to between 0.1-1Mb/s. When this happened, it would take a few hours for normal speeds to return.
But now we seem to be getting modem disconnects every night at various times (earliest has been 10:15PM, but often in the early morning hours), with connectivity resuming usually some time between 8AM-9AM. But now when connectivity returns, we have no degradation of speed. So that's progress, I guess. This has happened for the last 5 nights in a row so far. We don't seem to have this happening during the day.
When this first happened I got in contact with Spark, and they sent out some new line filters, which I installed. When this had no noticeable effect, they arranged for a Chorus technician to come out. When he came out, he found there was a problem with a connection of one of the external wires nearby (under a cover next to our neighbours property), and he said he had reseated it. He said he also went down to the Exchange and reset our port. When he left, our connection was up and performing at it's normal speed (it was down prior to him arriving). This was about 4 weeks ago.
As a next step I am thinking of replacing the modem with a Vigor DV130, to see if that helps us at all. Do you think this would be a worthwhile next step to take? And is this a decent enough modem to use? It will be paired with an Asus AC1900 router.
And yes, I know I probably need to get Spark involved again. But I'm wondering if it could be as simple as a modem that has served its time, and now needs to be retired. I actually tried call them that during our morning outage on Friday, but had to wait for a callback. When this happened 2 hours after my initial call, the automated system put me on hold again, and unfortunately I had to give up after a few minutes of this due to work commitments. And by the time I got the automated callback, normal transmission had resumed, so I was a little less motivated to stay on the line.
Here are the connection stats from my modem from when it is performing normally (12 Mb/s)

And here are the connection stats from my modem after an earlier disconnect, but with connection restored with vastly degraded performance (1 Mb/s)
