What is in your ftp logs?
Does it happen on other client machines, with other isp connections, other network cards in the machines , other routers.
See these they might help:
MTU packet size:
http://www.experts-exchange.com/Networking/Q_21626247.html
ftp buffersize:
http://www.developersdex.com/asp/message.asp?p=2675&r=4804367&page=2
Try changing the network card drivers and firmware for your router. What is your network card and routert?
Googling around others seem to have problems with (cisco linksys/kit) with 64k ftp PUTs (whereas yours is 128KB) so I suspect it is a client routing issue.
Good luck. :)