Dale,
Would you happen to have access to Crystal Reports 2008 (or anything Crystal Report 9.0 or higher)? They made some pretty significant changes to the way parameters and queries are built starting in Crystal Reports 9.0 and, while most users have no problems at all with backwards compatibility, this lingers in my mind as the most likely suspect for the issues here. Why it would work with viewing is puzzling, though. Unfortunately I don't have access to an AS400 to test this against with Visual Studio. You don't know of any sort of hosted AS400 I could connect to (maybe via VPN) to try to replicate the issue do you?
Thanks,
Adam