Steam game download speed drops while downloading?!?

I'm downloading fallout new vegas, and until about 75% it was at 2 MB/s but then it suddenly dropped to 200 KB/s - 500 KB/s?!?! I've noticed it ALWAYS does this with every game. No other downloads are happening either. what the hell? And i'm buying this game a long time after any DLC or the release date to the servers wouldn't suddenly become congested at 75%? HELP. Also it tells me to say the average speed of my cable in the settings. I have 20 meg broadband but the options are just different cables and i don't understand it. Please help!