What can cause a DSL connection to become inconsistent over weeks on end?

Some net companies "throttle" users when they reach max bandwidth, which may be causing your stalls when you're running multiple apps and DL's. It could also have something to do with your computer itself (virus?) or the physical connection of the wires to your house - if an exposed wire connector is rained on, it can corrode and seriously slow your signal, additionally any wires running near your line can "steal" some of its current through electromagnetism, reducing your signal. Is there a splitter before your router? A direct connection is your best bet. Hope this helps, and keep in mind that a lot of ISP's just suck.