surfing and download speed of internet

i have a problem in my airtel broadband connection....i explain you from starting.. my operating system is window 7 ,i start my computer open the browser window good surfing speed till now and now i go for downloading something download speed is same extent with my broadband connection...i am happy till now...downloading is on its way and now i go for surfing surfing speed slow , download speed decrements to 10,6,4,1 kbps bouncing speed ...my download speed from my connection is 256(means 256/8=30-32 in range)...but its bouncing above i mentioned....actually sometimes no its mostly happens whenever i open you tube or some other sites which have videos playing ,good at start (nothing happens to speed)...after that speed again goes to 30,20,10 kbps(from speed test browser window)...but it should be 256 or above ....between 10-15 minutes my speed test shows decrement speed and then if my restart my computer it goes to normal or after 10-15 minutes speed goes to its normal position(256 or above)....i have already complaint about my broadband service provider, he says format your window nothing happens ,same drama again..then he says what is your operating system on your connection i says window XP he again says format your window 7 to xp this time i cant understand what he is saying...is there any relationship between os and connection please somebody solves my problem......please help me.....