[Max Lobovsky]

550kB/s * 8 = 4.4 mb/s (thats megabits). You were exceeding the rated speed. I have seen this, too, on my 3mb/s connection and I think that cable companies give you a little more speed when few people are using the service. If the spike is only a second or two long, I believe it could be a result of your modem (and possibly router) caching data and releasing it in a large burst that exceeds the maximum speed.