@skyeci,
Thanks for your reply. It turned out that the D7800 does not provide detailed DSL stats. I found out that the line was connected to a Huawei cabinet, so I installed an unlocked HG612 which provided some useful stats and more importantly, the line is now completely stable with no dropouts. The mismatch of the flaky Lantiq chipset in the D7800 and the Broadcom chipset in the Huawei cabinet was likely to be the cause of the dropouts.
I returned the D7800 and bought the R7800 instead (the cheaper, router-only version of the 7800 series), made a little modification to add DHCP authentication for Sky, and now everything is working well with several weeks of stable uptime.
The R7800 has excellent third-party firmware options, whereas the D7800 has none.
Lessons learnt: Match chipsets and keep modem and router separate for best flexibility.