Windows 7: problem with my connection in windows seven only

after setup widows seven for the first time , its really good but there is a problem here with my connection

when i download something , ping become bad "as usual " but when i stop downloading, ping stay bad like im downloading " using IDM" , and even when i buffer something from youtube , connection stay bad and many "request time out" in my ping
and when i restart my pc, connection become so good as it was before downloading !
anyone knows what the problem ? and how i can fix it, thx alot