DSL slower?

I noticed a dramtic slowing down with my DSL connection, I started to run Verizon speed test: http://infospeed.verizon.net/index.asp everyday monitoring my connection. The service I supposed to get is 3MB, however, during the days, my connection, according the the speed test, dropped down to something like 56K/126k/256k in variations. I started to spend large amount of time with Verizon DSL Tech Support. And eventually, after my case was full of remarks, Verizon’s first tier and second tier support had enough with me…refused to give me help any more, asked me to take my computers back to the manufactures. Right now at 2/3 am in the morning, my DSL speed is excellent, my speed test is showing a perfect score, 3MB! Does DSL have traffics like the dail-ups?What happened? Does it something to do with Verizon/Yahoo deal that changed some configuration from their end?