Hi
I'd agree your drop sounds quite normal.
Upstream uses lower frequencies that travel very well and uses much fewer of them so overall as a target for noise, they don't get hit or affected very much so you normally see much less variation in SNRM. For my line on Be I use a 3.5 db profile so I get a bit extra of upstream as that runs at 3.5db which doesn't change much, then tweak the downstream SNR locally so it runs at around 7db as it would normally to give enough margin for the swings.
On the downstream what will cause SNR fluctuation is the higher frequencies, these are attenuated much more, and are spread out over a bigger chunk of frequency space so get hit by all sorts of noise as night approaches. This is one reason while when on ADSL1 and a fixed line rate say 1 or 2Meg, which on most lines the higher frequencies are not needed as there is enough capacity in the lower range, SNR swings were much less, unless you were on a very long line.
Switching to ADSL2+ which unless you live next door the exchange means your line is working at it's absolute maximum, SNRM starts moving around a lot more.
Regards
Phil