Auto buffer size related to slow download?

[inetscan]

I'm noticing that about 1 in 5 downloads (from the same ftp, same types of files and sizes) will not get above 70KB/s while the remaining downloads have no problem staying between 2-5MB/s. A quick abort and transfer usually corrects the slow transfer. So I'm curious if the auto buffer size is potentially causing the slow transfers. When and how often does the tcp/ip buffer size change when set to "(auto)"? Is there any way to see the value of the buffer when it is being automatically configured? Any other idea on what might be causing this or how to diagnose? Thanks!