The first things to check if you have an unstable line where the connection speed can vary a lot, or you get frequent disconnections (and/or on BT-based connections varying IP Profiles) are:-
1 - Check the phone Master Socket for the presence of a ring/bell wire and sort that out if there is one. It can reduce connection speed by over 2Mbps, though 500-1500kbps is more common.
2 - Make sure absolutely everything that connects to the telephone line goes through a working (not faulty) filter. This includes Sky boxes and phone-connected burglar alarms.
3 - In the case of Sky boxes it seems some are so noisy that double-filtering is needed. So a filter into the phone socket, a second filter into the first one, and the Sky box into that.
4 - Many people like to make neat coils of the myriad wires everywhere, possibly tying them together first. Doing that creates a magnetic field which can be bad news. A random jumble is usually better although unsightly, and separation of signal cables, power cables and power boxes/transformers/switched power supplies (Including those built into the plug) is wise.
No prizes for pointing out that separating the power cable from its power box/brick isn’t helpful.
Never run signal cables and power cables parallel to each other and very close together as there is a magnetic field created around the power lead anyway. If they have to cross do it as near as possible at right-angles.
Always keep signal cables well away from power bricks and transformer type plugs.
5 - See also Miscellaneous nasties for whatever less common problems occur. Updated as I come across them.