Could be a lot of things. Unreliable download sources. Your buddies website might be on a crappy connection and you're losing packets, Router is losing DHCP lease or whatever, faulty install of NIC drivers, your ISP, there ISp... how long have you been having this problem, try restarting your modem. I'd just wait it out.
theres really no, "This is your problem" answer i can think of.