Connection speed sporadic. Help?

For some time I have been having issues with my internet connection. The problem lies with downloading files as well as buffering videos. My internet tends to "time out" in the middle of downloads or videos and I always have to restart them or fast forward to get them to start buffering/downloading again. I have no problems browsing the internet or playing online games, just long extended uses of bandwidth always tend to freeze up.

I am running on a wired desktop using windows 7. The computer is fairly new so slow hardware should not be causing the problems. I have also confirmed the same issues while using my wireless laptop. And my internet service provider is att Uverse in which we have the 18mb/s download package. Here are the specs for the desktop in case they come in handy.

Windows 7 Ultimate
Gigabyte GA-P67A-UD4-B3
intel i5-2500k sandy
16gb ram
amd radeon 6970
750 watt power
500gb internal 1tb external

Please, give me a hand with this