Networking :: File Truncation - Transfer Interruption With Outside Networks

I do IT work for a decently-sized bookstore. Over the past year or so we have had a recurring problem crop up where file transfers with networks outside of our building will be interrupted, or those receiving our uploads will get truncated files. It is an intermittent problem and I have not been able to narrow it down to any particular day of the month or time of day. Files that are dealt with in our web browsers have no issues - I can happily download and upload large files all day without interruption. Yet when our little postage meter machine tries to connect to its update server, it sometimes has its connection interrupted downloading its update files. Our credit card machines sometimes have trouble holding a connection long enough to batch out, and our storewide POS system often has to resend its credit batch multiple times before it can maintain connection long enough to push the whole file through. One server daily ftp's a ~11mb file to an associate, and a few days a month they will receive a truncated file that appears to have been cut off mid-transfer. Our bandwidth is handled through three T1 lines that share phone line and internet service. Our voice calls never get dropped and have good line quality, but mobile credit card terminals will have trouble maintaining a connection when they dial out.

I'm thinking this is either an ISP issue or an internal network problem. I don't know what protocol the mobile machines use, but it's possible it could be restricted to ftp or, I dunno, telnet? Maybe a port forwarding conflict? Files sent internally have no problems, so I'm thinking it's something at the firewall/proxy/router stage.

Our servers run a number of *nixes: our central databasing/pos is handled by an IBM RISC box running AIX 4.3; our DHCP server runs Debian Sarge 3.1; we have a Ubuntu 8.04 LTS box running a squidproxy setup using squidguard for url filtering; firewalling is handled by a Debian 5 box running Untangle. Our ISP provides us a 4.5 Mb connection via three T1 lines that provide shared bandwidth for our digital telephone lines and internet.