Trouble using ftp through cisco

I have been having some intermittant problems using ftp on various computers. We have a cisco router. It will connect t the site, but then the problems start. In active mode, it gives the error 425 can't open data connection when doing MLSD. When using pasive mode, it gives a 421, can't create socket and then a timeout. Not sure if this is a router issue, but if I use an air card with the same computer, it works fine. It also works fine on another network. The computer is running windows xp and we are using filezilla. We also tried the comand line and it hangs when doing a directory listing. Any soggestions on what is wrong or how to troubleshoot this. We finally have a duplicatable situation.