Are you archiving, dubbing, printing??
Are you using a software based editor? (final cut, adobe)
It's always a matter of settings when 'letterboxing' is used.
Are you printing to tape from the computer or from a camera?
Sounds like your box is trying to cram 720x480 into 640x480.
Let me do some digging and I'll try to see what you're up against.
Keep us posted in the mean time!