Why Vista's networking speed varies?

I've been transferring a bunch of data between several NAS's on our LAN.
Nothing else significant is running on my Vista system -- just a few
background processes using 10% or less of the CPU.

The transfer varies wildly in speed. It might run for a while at "130%
network utilization" according to the Performance Monitor (I've seen over
130Mbps on my 100Mbps LAN!?), then for no apparent reason it will drop down
to 50%, 10%, or even 1%. The total B/min shown in the Perf Monitor drops
from nearly 500 MB/min to as little as 5-10 MB/min.

Why?? What is causing Vista to not use its resources to do this transfer?
When the transfer rate drops, the CPU is still 60-70% idle, just like it
was when the transfer was fast. There is no indication of any unusual
activity in disk accesses or memory hits. It just looks like Vista gets
"lazy" and doesn't want to work that hard, so it slows down until it feels
like going fast again. I suppose it's possible that the NAS is slowing
down the transfer, but that seems unlikely. It's just a tiny lean Linux
box with nothing else to do. I've seen this talking to both WD and LaCie
NAS boxes.

Any guesses why it does this, and how I can keep it working at full
efficiency?

Thanks,
Gary