Mysterious download surges during torrenting

My system:
Compaq Presario 2500
704 MB RAM
27.9 GB HD (823 MB free)
XP
µTorrent
Seagate FreeAgent 500 GB USB-HD
10.0 Mbit/s

The mystery:
Every minute there's this huge-ass surge in download speed. While the average download speed is some 250-300 kB/s, the surges could be as high as 6000 kB/s. Is there a bottleneck in my system (as the USB connection to the HD) or does µTorrent work in that way?

Please help me solve this mystery, beacuse my client is clearly under-achieving.

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