Unreliable / slow internet

[Eleandor]

The past 6 months I have been experiencing many frequent disconnects. I tried a lot already, but now I'm really getting tired of trying on my own.
On top of that, when I'm actually connected, my internet is really slow. Downloading a file faster than 100kb/s is rare here. Given that I should have 12mbps this sounds rather weird.
Basically, the main problem is that I constantly disconnect from the internet. It's very annoying for daily webbrowsing / checking email, and online gaming is pretty much impossible.

My current setup:
Sitecom ADSL2+ Modem/Router 54G Turbo WL-174 at the 2nd floor in my house
3 phones, each connected to an adsl filter. Sometimes (although rarely) the phones stop working too.
1 desktop connecting through ethernet, 1 desktop & 1 laptop connecting wireless.
The connection: Belgacom adsl go - 12mbps down, 1mbps up. I live in a large city.

I have verified that the problem is internet-related. I have no problems with the wireless network. Here's what happens:
The "adsl" led on my modem suddenly turns off. A couple of seconds later it starts blinking until it's turned green again. Next, 9 times out of 10 it turns off again and the process repeats. Otherwise, it stays green for a couple of hours, occasionally 2 days. But most of the time it just keeps connecting and disconnecting in a loop until it gets somewhat stable again.

I have already contacted my ISP, they say there's no problem on their side, and the problem lies with my modem. Bullshit, because after the call I have replaced it several times with other modems. My neighbours have the same connection with the same ISP, and they have no problems. Using their modem does not solve the problem. Tried recontacting them several times, but their helpdesk service is truly horrible. Currently I'm using the sitecom.

The adsl status on the webinterface of the modem reads this:
SNR Margin: 15.5 db Downstream / 21.0 db Upstream
Line Attenuation: 40.0 db Downstream / 19.0 db Upstream
Data Rate: 4640 kbps Downstream / 512 kbps Upstream

I noticed that when my modem is stuck in a disconnect/connect loop again, the SNR Margin is very low (4 to 8 db). At this moment it's pretty stable.
One of the things I've already tried doing is moving the modem to the ground floor, which should improve the line attenuation. But the numbers remain the same...

It's extremely annoying and I have no idea what's wrong. I may contact my ISP in a shop this week. But meanwhile I'd like some help from other people. Thank you.