I don't know what is causing the resyncs, any tips for things to track that could help diagnose?
When, and how often, are the resyncs?
Resyncing in the course of a day is unusual for VDSL2, even when a line has problems. However, DLM can get involved because of issues with an increased error rate, or an increased rate of resyncs; any intervention by DLM then usually causes a resync too (usually between midnight and 8am) ... but that one obviously isn't the root cause of the issue, and is more a consequence.
A resync that isn't ordered by DLM, or triggered by a power cycle or physical disconnection, is then likely to be caused by noise on the phone line ... though this is noise in the VDSL2 spectrum, up to 17MHz, rather than noise you can hear. And, often, the biggest source of noise is the inadvertent crosstalk from your neighbours also running VDSL2 connections.
Over time, then, noise increases, SNRM levels drop, and the speed attained after a new resync will drop slightly. Sometimes you might drop just 1 or 2 Mbps, sometimes it could be 10Mbps for a particularly bad disturber. This is the usual picture for normal, working lines.
It is unusual for noise to decrease, and speeds to increase, unless one of your disturbers turns their modem off for a while. Someone who regularly turns their modem off each evening, and back on again in the morning, is creating havoc for some poor neighbouring line.
Your symptoms don't quite fit with that pattern - the drop of 25Mbps seems to be too much for one disturber. And your "Cur SNR margin" values of 5dB each time go against this too, but they are only limited snapshot figures.
Other causes can include EMI induced onto the line from sewage pumps, electric fences, etc.
To get a better picture of whether something like this is happening to your line, or whether you are the victim of high error rates, you really need to have the statistics being gathered into graphs, such as through DSLstats. Being able to visualise changes in SNRM, attainable speed, actual speed, FEC error rates, CRC error rates and ES counts all improves the advice you can be given.
Like @burakkucat, I find it hard to read the Draytek output, and find that most of the useful data is missing.
@banger I don't use the telephone phone for phone calls. What's the best way to test for noise, just plug in a telephone and listen?
Yes - if you are using a BTW voice line, then calling 17070 and selecting option 2 puts the line into a "quiet line test" mode, but essentially, you are using your ears to figure whether there is anything crackling, rustling or humming.
The theory is that if there's audible noise, then there is likely a line fault that creates noise that affects VDSL2 too.