strange errors, need help

hi,
i'm running a rather large website with a couple of thousends users per day.
almost every page does some ado.
now and then im getting the following errors:

System.InvalidOperationException: Internal connection fatal error. at
System.Data.SqlClient.TdsParser.Run(RunBehavior runBehavior, SqlCommand
cmdHandler, SqlDataReader dataStream, BulkCopySimpleResultSet
bulkCopyHandler, TdsParserStateObject stateObj)

and

System.InvalidOperationException: Invalid attempt to call FieldCount when
reader is closed. at System.Data.SqlClient.SqlDataReader.get_FieldCount()

the weird thing is: the statements are ok, next time they are called, they
might run just fine.

Any ideas?

regards,
michael