If your ADSL internet is slow or dropping out, and/or if your phone line is crackling (and perhaps you lose dial tone when it rains?) then you could have corrosion in your phone sockets like in this photo (see the green/blue coating on the screws and the crusty bits on the contacts? It is copper "rust", not orange/red like rusty iron, but its the same oxidation chemical reaction (between copper & oxygen) in the presence of water).
This is a old Australian 610 type phone socket in a bottom-floor apartment, in a suburb not too far from the coast. This is common in units/flats/apartments as the ventilation (in older buildings) is usually not too good. How your line is affected depends on how bad the corrosion is and even the humidity in the room.
Corrosion between metallic contacts, terminations and wires causes a change in impedance which affects both Voice signals (by inducing crackling noises on the line, sometimes it can even short circuit the phone line which drops your call and drops dial tone so you can't make calls) and ADSL data signals (by causing the same noise on a specific "channel" or range of frequencies that causes dropped packets / error packets, or causes the modem to mark those channels/frequencies as "too noisy" reducing your overall bandwidth - reducing your sync speed, and ultimately, your data throughput).
Please note this is not limited to ADSL or ADSL2+, corrosion can negatively affect all copper line communications including VDSL, SHDSL, EoC, etc.