Download speeds increasing to 5,000+ KB/s intermittently

[CaligulaX1]

This is my first post, and also my first encounter with my download speeds increasing to such high speeds. I have Comcast Broadband and my usual download speeds for Limewire usually don't exceed 450 KB/s. Today, my average speeds are around 800 KB/s, but now the speeds are intermittently increasing to 1,250 KB/s ---> approx 3,000 KB/s ---> up to 6,652 KB/s, but only for a few seconds and then back down to around 800 KB/s. When this happens, I notice that the download speeds for the current downloads don't increase at all. Also, I notice that the Upload speeds box at the bottom says that there are "0 @ 1 (goes up to 5) KB/s at the same time the download speeds are jumping to 5,000, and when there are no active uploads occurring at this time. I am afraid that this might be the work of a virus...any thoughts? I haven't opened any files I have downloaded as I always scan them before I ever run them on my PC.
Thanks for your input and advice!!