I don't believe that your problem lies with ad- or spyware, this is actually the classic signs of your modem or router going out actually.

DSL is a very unstable connection to begin with -- is it raining outside right now? It is unusually hot or cold?? Are you up or down a hill?? All these things can affect your internet speed when it comes to DSL.

If the problem continues the ONLY suggestion I have is to call your customer support. Like someone above me said though OF COURSE they will never admit to it being a problem on their end, its ALWAYS the customers' fault

Good luck though!