I'm sorry you've run into this issue. When you say 300 fields, do you mean that your database mapping consists of 300 fields mapped to the SQL tables? DAT opens in InfoPath, and there are some performance limits. It sounds like that's exactly what happening in your scenario, with the tool freezing.
Page 22 of this document provides a manual workaround to editing large mappings, but please let us know if we can help further. Thanks.