Guys I've found THE CAUSE of my SNR drops and CRC errors. Everytime the elevator is called, it happens.
Now I understand why the connection is better at night.
Now I'll wait for Sodetel's outdoor technical support to show up and see what they'll conclude.
Technical insights: back when my speed sync speed was at 4Mbps, I used to get little CRC errors which didn't affect the stability of the connection.
According to this source (
http://www.increasebroadbandspeed.co.uk/graph-ADSL-speed-versus-line-loss-distance), if your current SNR target margin is 12 dB and you decrease this by 9 dB to 3 dB, this will provide a boost in downlink connection speed equivalent to a 9 dB reduction in line loss.
This means that at 4Mbps and 44dB line attenuation, my SNR used to be 16dB given these information:
1) Target SNR by ISP is 6dB.
2) My line attenuation on ADSL2+ is 44dB.
3) According to the source, (1) and (2) should give me a speed of 7.5Mbps.
4) My speed was fixed at 4Mbps, which is at 54dB line attenuation according to the source.
5) 1dB decrease in line attenuation is 1dB increase in SNR margin. My line attenuation is 10dB less than the minimum required for 4Mbps. So these 10dB go as a plus for SNR. My SNR is hence 6dB+(54dB-44dB)=16dB.
6) When the elevator is called, the SNR drops by around 8dB. 16dB-8dB=8dB. So at 4Mbps my SNR with the elevator's electrical interference is 8dB which is above 6dB which is why I didn't lose sync.
7) When I called Sodetel about my problem they fixed my SNR at 12dB. With 8dB decrease in SNR in case of interference, the SNR gets down to 4dB - which I noticed - and that causes the CRC errors to boom and the connection to hang.
8) I'll request Sodetel to fix my SNR at 6+8=14dB+1 extra dB=15dB.
9) 12dB current SNR + 3dB to get to 15dB => 3dB increase in line attenuation. This means 6.3Mbps max rate => Around 4Mbps actual downstream rate (because now at 7Mbps max rate I get 5Mbps, so I'm expecting 2Mbps actual rate less than max rate).
Hopefully this helps someone somewhere.
Update: if you're reading this in the future, note that it might not actually be the ISP who set my SNR at 12dB. It might have happened magically after I called them several times and asked them about it.