[CentOS] vsftp

I'm having an odd behavior with vsftp on all CentOS 4 servers... maybe 3 
too, but I'm not on those much.

I am transferring in port mode as I've never managed to get passive to 
work with a firewall running.

Anyway, this is what happens.

During normal transfers with few files.. it works perfectly.

During the transfer of like a large website with hundreds of files in 
various directories, from a fast connection, vsftp stalls out with a 
high degree of consistency. I thought this was happening only when I was 
doing this locally through the intranet, but I have now discovered our 
outside users are having the same problem when they are on a hi speed 
connection.

Does anyone have any ideas as to why this is happening?

Thanks,
John Hinton