one reason could be that the T1 host has too many people uploading at one time.
i have T1, i allow up to 11 up-loads at a time. generally set at 9 first thing, then adjust as needed.
if i have alot of slow movers (1-5KB) i allow more(from 1KB to 35KB each).
if i have all fast movers (15+KB) i allow less at a time(up to 20KB each).