hanging ip-stack/ overload of files

When increasing the numbers of threads higher than 6 (windows prof xp), and there are a lot of small files,
after some time the internal ip-stack of smartftp hangs. the download is pending, and no possibility to reactivate.
Only rebooting the os makes it possible to open new connections. Smartftp gives the message:
connect error. but using another ftp- or sshclient, no problem.

typical value of downloadrate is about 4 mb/s, internal network (company) is gigabite (1000mbit).
the servers are i- and p-series with up to 12 cpu's, so there should be no problem of resources

another effect is reopening the connection after pending, but "overloading" the file.
the original file has e.g. 10 mb, the download increases higher and higher, and doesn't stop.
So, after a while, you have something about -> what you want. sometimes it breaks after double-size,
sometimes ist increases further.
This kind of a resume of the paused download isn't the one which is really useful .

to be on the sure side, I configured max 6 threads. each threads has typical up to 500 to 600 kb/s.

the same behaviour occurs for example using edonkey behind a router, with to much connections, and the router will hang -> overflow of internal ip-tables -> please reboot .....
-->>So, it would be nice, to implement an internal "dynamic load balancer", which is able to increase/ decrease
the number of threads due to the absolute number of changes of the connections.
i tried it manually, but having a queue with thousand of files its a little bit wasting of time ;-).
-->> it would be fine to implement this. so you can use it in a professional enviroment and not
only at home .