bizarre-internet slow and can't access on demand

  WVMMRH 10:12 25 Apr 2006

i've had comcast cabel modem for several months and suddenly something strange is internet surfing has sloweed to a crawl and at the same time i can't access on demand for all the pixling and jumping(like a poor incoming signal).i recieve all the other channels fine however..local and premium.
this cleared up yesterday and i was recieving on demand clearly and my internet was fast again..tonight it's all gone weird again. i recieve the local and premium channels great.but on demand is pixling out bad and my internet surfing is slowed to a crawl. even google takes about 12 -15 seconds to fully load.this all began happening almost a week ago..this is the fifth day. comcast claims it's on my end or inbetween.certainly i can't be the only person that this is happening to.and why does on demand fark out at the same time my internet does? one of my friends said that he believes that it might be happening at the outside connections on the building(i live in an apartment).he thinks someone in my building might be trying to steal or share my connection. does anyone have any idea as to what might be going on?i appreciate any advice,etc.

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