WAN users download files extremely slow…

Help! I have been having issues lately with network throughput on any file that is accessed from an off-site location. Currently I have a Leopard server (running DNS, Firewall, NAT, VPN, iChat) that is providing authentication services to my users who are logging in from an off-site location. I also have a second server running Tiger server that is set up as an OD Master running AFP and providing authorization services to my users. This is where most of the files my users are accessing are located. I have run speed test after speed test on our internet connection that consistently yields a rock steady 337KB/sec UL and 319KB/Sec DL (We have bonded T1's @ ~3.072 Mbps) yet when users are trying to download or access files using AFP or FTP the server will sputter between 0-120KB/Sec and never achieve anywhere close to its maximum theoretical throughout. I have tried to confirm in as many ways as possible that this is a file service problem and not an internet connection issue. I regularly use Remote Desktop from my home to connect to the server that yields bursts and sustained speeds all the way up to 340KB/Sec. Again, bursts are not saying much, however when the server was displaying a screen saver in full color mode, the throughput was steady at ~3.072 Mbps. Furthering my suspicion that this is a service issue, I have set up a VPN that sends all traffic over the VPN and then logged in and visited Youtube and other download sites, all yielding the expected maximum 3.072 Mbps speeds when downloading files from the internet to my computer. In order to try and narrow down the problem further, I have turned off the VPN service (thinking that perhaps the overhead for encryption for multiple users was causing the low throughput) and tried both secure and insecure AFP and FTP downloads from the server. Again, this yielded the same pathetic throughput. Is there some issue with transferring files over the internet that I'm not aware of? I'm literally tearing my hair out trying to figure out where the problem is. Users are taking anywhere from 2-5 minutes to open small (