We all just need to become engineers and we'd never have DLM issues again.
A far better option would be to give the ISP ability to set certain parameters - its only the NGA DLM that is so rigid with no configuration options available.
20CN/21CN and other ISP DLMs give flexibility such as Interleave On/Off. Target SNRM & INP level options are also available to ISPs such as TT.
In BlackSheeps case rather than having a full reset which 'cost' 20Mbps, it would perhaps have been better if he could have limited the target SNRM to not drop below say 5dB. From BS's comments it would appear the line behaved perfectly fine with G.INP until it got a target SNRM of 3dB.
Both Dray and Jelv are correct.
Drays suggestion of capping the line would probably work in BlackSheeps case whereby he had no problems until he got the lower Target SNRm
Based on a previous sync of 60Mbps at a Target SNRM of 3dB, but setting a max sync speed of 50Mbps in the modem that would increase the SNRm and ensure that the line didn't fall below 6dB.
Jelv is correct that 80/40 etc are artificial caps that suffice to increase the SNRm. However Jelv's issue with the Central Heating being switch on/off sounds like it was possibly SHINE rather than REIN.
G.INP doesn't work well on all types of noise which is why Openreach still has Interleaving as a backup if G.INP doesnt work out.
The reason why the Zyxel has probably worked so well for Jelv is that unlike most other modems it has an internal noise filter.
Chrys also makes a valid point, by performing a reset eventually G.INP will be re-applied and the line will resettle back at around 50Mbps. Historically BS's line has always worked well at 6dB and DLM will be happy. So happy that it will then start reducing the target SNRm again. When it gets to 3dB then BS wont be too happy because he sees some line drops. If he resets DLM at this point then you can get into a fighting the DLM cycle.
If left alone for a while DLM should in theory spot those line drops and the Errors that no doubt will come with it, it should then increase the target SNRm by itself to a more stable level.
All this messing around could so easily be avoided if Openreach gave some control with the params.
I'm currently having a fight with DLM, you will see just the other day yet again I had to turn my modem off. This is because 99.9% of the time it works fine at default, but every few days or so there is a massive surge in errors that triggers DLM penalisation of 10Mbps. Yet by the time DLM spots it and takes action the line is already back to normal and doesnt need DLM action.