when you say "back-up again on the same rate", are you implying that for the first 18-20 minutes you achieve a high bandwidth on everywhere on your network, and then after that time span, it reduces to a bare minimum??
or am i misunderstanding that phrase?
use a single pc to hook up with the adsl modem. monitor bandwidth for an extended period of time. then go back to your original network structure. check each and every pc that is connected to your switch/hub. check bandwidth simultaneously on each pc for that same amount of time. check which pc or application is creating any bottleneck if there is any. let us know of your results