A few of things can generally cause that.
1) A route change on the Internet by your ISP, usually due to a different backbone, or newly installed backbone connection or a bad route propagation.
2) Your computer getting hammered with broadcast floods, worm attacks, possible local DLL's running due to spyware or trojans.
3) Your ISP backbone getting saturated, either due to abnormally high traffic, or DoS (denial of service) attacks.