I had a similar problem before
Some Internet service providers give a misarable bandwidth for some internet protocols, and could cause 1 program to eat the bandwidth of the other. in my case, i was unable to use utorrent and World of warcraft at same time.
So, first thing to try is to close any other program running on background from all computers, specially those who use your bandwidth.
Check is a evil neighbor or outsider is stealing your wireless bandwidth (today happened to me
)
try using a cable instead of Wifi (just like a try)
If you are using a router, try to conect your laptop directly to the modem. Reset your router config this worked.
uh... I don't have any more ideas for now.