For a reliable ADSL connection at a specific speed, it's not just the noise margin that's important, it's the downstream and upstream attenuation (particularly downstream). For 2M bit operation, the theoretical breakpoint for downstream attenuation is 43dB (the lower the number, the better the situation). Sometimes, however, if the noise margin figure is sufficiently high, you can still get reliable 2M bit operation at a worse attenuation figure than 43dB. For instance, I myself run happily at 47.5dB attenuation, as reported by my modem. Beyond 50dB attenuation, though, you're very unlikely to get reliable 2M bit operation. Incidentally, the figures you get from your modem will only be ballpark ones; they're not meant to be absolutely accurate.
You desperately need to update your modem's firmware and then run its WAN diagnostics, to get the downstream and upstream figures for attenuation and noise margin. Without knowing these, you'll be going nowhere. It might well transpire that your attenuation figure will be too high for reliable operation at 2M bits and that's why you've been getting dropouts. You might therefore have to revert to 1M bit operation instead. You probably already appreciate that the further you're situated from your local exchange, the higher will be the attenuation.
Do follow that yarwell link, as that person's a valuable contributor to the
www.adslguide.org.uk website's Technical section. It sounds as if you need to disconnect the Ring wire on your extensions and to generally tidy up your extensions. Run them only as daisy-chained connections, eliminating spurs, as that's the only way you'll optimise the setup for speed. And ensure you use a plug-in filter only where it's truly needed, ie. where a phone is also plugged in.
From personal experience, a Belkin high-quality ADSL extension lead will help (due to the better twisting of the pairs in it) but don't bother with a so-called screened lead, as the degree of screening is actually minimal (it'll only be fully screened if the braid is connected to earth in the modem or the BT socket, which it won't be). EBuyer do those Belkins. Choose carefully, as some have RJ11s both ends and some have RJ11 one end, BT at other (avoid unnecessary plug-in filters).
Plug-in filters vary in their performance as well. Personally, I've found the cheap-and-cheerful ones from Solwise (
www.solwise.co.uk), with built-in Ring reconstruction, to be perfectly adequate.