Strange changes in SNRM on BT Infinity 2

Hi, just wondering if anyone can shed any light on the following.
I upgraded from ADSL+ to Infinity 2 early February and was one of the first to be
connected in the new cabinet.
Initially I had a max attainable speed of 84Mbps down at 6db SNRM and a throughput speed test of 73Mbps.
About 2 weeks after installation the SNRM of 6db only gave max attain of 81Mbps
which has been dropping ever since and is now at 69Mbps after another drop today.
The strange thing is that the last twice I have been monitoring it and the drop in
SNRM has been instant and about 1.2db in each case not the gradual up and
down you get with day to night and back to day. First time was at Monday at 9am,
second time was today at 4:30pm.
Its as if the DSLAM is reducing its TX power, anyone know if this happens, maybe
to reduce crosstalk as more lines are connected.
No re-sync occurs when this drop happens but the margin to operate is reduced
and a re-sync is sure to occur as it gets worse and closer to the present operating speed.
I have done all the usual quite line tests, BT speed test, restart of equipment etc
but nothing appears to stop these big instant jumps in SNRM
At the current rate I will be down to infinity 1 rates within a couple of months !
P.S. I am approx. 500m from the cabinet and the wholesale checker says I should
be getting between 55 and 65 Mbps so at present I am within the given value, just.

This could be increased crosstalk due to increased takeup on your cabinet.

Almost certainly this is increased cross-talk. I was one of the earlier adopters on my local cabinet (700m cable length) and had 57mbps and now it's at 50mbps. I saw something similar with ADSL. It will stabilise, and I don't think it will get anywhere near the 40mbps level - 500m is quite a distance to get 80mbps in the first place. I suspect that you'll eventually end up with something in the 60mbps region.

This is the sort of thing that vectoring is meant to help with, although I wouldn't hold my breath over it as it needs more investment and a retro-fit might be quite complex.

On other thing is that I believe the ANFP profile for VDSL2 provides for power levels a little below what is theoretically allowed by the standard. It might be a power boost might help, but possibly there would also be downsides.

Thanks for those comments I was thinking this could be crosstalk issues.
The other problem is that the DLM has slapped INP 3, delay 8ms and interleave
of around 1300 on the link so a max attain rate of say 69Mbps as it is now results in a sync rate of only 60Mbps and a throughput of around 58Mbps at best, with a ping around 30ms into the bargain.
Hope it settles down at this rate and does not get any worse.
Guess if I had not known it could go faster I would be less bothered.
Roll on vector support which should help but guess we will not be getting this
until late next year or even 2016 !
(Just seen TBB news that Ireland are to get it now, come on BT follow that)

Interleaving does typically drop the sync by 10Mbps or so.