Unlikely to be a network problem. SQL's behaviour on data modifications is all or nothing. If an insert fails, it fails, it doesn't succeed and leave some columns NULL.
Check that the correct statements are being issued to the DB (Server-side traces, since you're using SQL 2008). Check that no one's modifying things after the fact (triggers on the tables)
Gail Shaw
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability