One thing comes to mind.
Hop on over to
www.dslreports.com , make a free account, go to tools, run the 'tweak test.' If need be, you may need to change your MTU and RWIN. A bad RWIN can cause lags in data transfer.
Just a thought.
-SF
Oh, and remember, if you're running a PPPOE on a router, or hell, even a router as a gateway, you may need to change the RWIN and MTU of your router.
Oh, and are you running a router?