Don't you mean the other way round?
My QLN is around minus 140dB, in your 90Mb graph it is around minus 140 & your 73Mb graph it is minus 120.
Minus 140 is quieter than minus 120 (i.e noise is more attenuated).
Also, remember that QLN & Hlog are only updated at a resync & that data remains static until the next resync.
Bitloading, attainable rates & SNR do continually fluctuate during the whole duration an ongoing connection though.
I also see that Retrain Reason is 1 for your 73Mb graphs.
On various users' connections I only usually see reasons 0 (reboot/power cycle) & 2 ("on the fly" resync - usually when DLM adjusts things).
I admit that I don't really know what reason 1 signifies, but I suspect it may be due to a "problem", rather than a DLM adjustment due to crosstalk.
On the very rare occasions where I have seen reason 1, it has been on connections needing some repair/adjustment of either external or internal cabling.
Once that work has been done, those connections have tended to stabilise at higher rates.
I just wonder if someone might have switched on their Christmas tree lights using a "dodgy" power supply yesterday.
Do you have any idea what time the reync happened yesterday & did it coincide with another user being connected?
It is just slightly feasible that the engineer may have unwittingly "disturbed" your connection during that installation.
It just seems too big a drop for it to be crosstalk.
reason 1 is the retrain from when the attainable sync took a 20mbit dump.
Obviously if I am synced at 79999 and the attainable sync drops to 73mbit the sync will drop :p
since its been synced at 73mbit there has been no drop and likewise before was no drop.
So to make it clear there is no ongoing issues with the line, the line is actually very stable, I am still on fast path so DLM also sees no issue with it either.
The QLN test, a higher graph is more not less noise. It even states this on the hg612 unlocking firmware site.
So to brief again. The line has a very stable sync rate outside of 2 sudden rate changes that both applied early morning around 10am a week apart that took large parts of the signal away. It even barely variates from day to night.
The snr graph is also telling my 73mbit graph looks like its a new different line, the pattern is different the signal quality noticebly dips like its higher attenuation, it all looks pointing to crosstalk to me.