hmm sounds like auto update problem to me ... Go to your games selection screen and manualy disable (by rightclicking each game and selecting properties) the "keep up this game up to date" feature . That way , you'll be able to play your online game , without steam downloading content in the backround

Also don't forget to set the corresponding rates for your connection type , for example :
For adsl 384k users good rates are
rate 7500 - 9000
cl_cmdrate 45
cl_updaterate 45
To find the proper values for your connection type you should check the tweak guides at Cs Central (Also known as Banana Central) and CS-Nation .
Hope that helps !