You can use tools such as WhatsupGold to monitor your connection (it pings the ip, and shows when it wasn't pingable. Just ping your gateway, and you'll know)
DSL goes over the phone network, which is copper. It depends on the quality of copper, if your DSL connection is ok. Next to that the speed you can get depends on how far away your house is from the phone exchange box. Some telco's claim to give you 8 Mb but when your house is too far away, and the copper is not good enough, you won't get it.