ADSL is a proven, reliable technology, and on the whole it delivers a decent service for its users almost all of the time. When things go wrong, however, don't automatically assume that it's the line at fault.
Typically, a completely dead line (ie, no ADSL connection) can be attributed to problems at the exchange or on the phone line itself. Anything more transient than that, for example slow page loads or trouble connecting to random sites, and your router may well be the problem. The symptoms I experienced were:
My natural inclination was to blame my ISP (Zen Internet), but their reliability record is so good that I couldn't believe that they were dropping my traffic. Equally, the connection issues were so sporadic that I found it hard to believe that my router could be at fault. Routers do die eventually, so I decided to check if that was the problem after all. So, I've got hold of a new router (actually, an old one from work - a Draytek Vigor 2800) and the problems have immediately disappeared. My old router (a Belkin ADSL2+ device, probably over 6 years old) is now consigned to the 'emergency spares' bin.