Ok, since it's not easy to narrow down to the connection, try these other things first.
1) Make sure not overclocked
2) Modify your client.ini to the barebones posted here.
[Defaults]
WindowedModeXOffset=1
WindowedModeYOffset=1
WindowedMode=TRUE
[VideoMode]
Width=1024
Height=768
WidthWindowed=1024
HeightWindowed=768
WindowedWidth=1024
WindowedHeight=768
FullscreenBitsPerPixel=32
FullscreenRefreshRate=0
3) What kind of processor? Some problems with AMD X2 dual core processors can be fixed with the following official release:
http://support.amd.com/us/Pages/dyna...42f&ItemID=153