That is because you are using a cable connection it will change based on how many people are using the cable line, so at around 5-8 when everyone is watching tv/playing wow ect your speed is going to go down since the line is shared with those other services at night when everyone is sleeping and have there computers/tv's off it will increase since it's not a dedicated circuit like DSL/T1 or a FOIS line, that is also why most cable company's dislike torrents since it affects all the other services they provide on the line.
Kick ass upload speed btw.
also for all the people that switched to Comcast from Verizon... Verizon owns 50% of Comcast >.>
Being the adventures of a young man ... who couldn't resist pretty girls ... or a bit of the old ultra-violence ... went to jail, was re-conditioned ... and came out a different young man ... or was he ? - A Clockwork Orange