Linked Oracle Server invalid Binary Data sort dependent

Hi, I have SQL Server 2016 SP1 with a linked server Oracle 12c Linked Server. Most queries work fine, but there is an issue with Image data. In some records the binary data is corrupt. I changed from Oracle LOB to CLOB and this works with most entries, but there is an issue with some rows. If I change the sort order, **different rows** are corrupt and the originally corrupt rows are correct. I cant post full details, but one symptom is the magic number for JPEGs is removed from the binary data in invalid rows. Changing the sort order alters which rows are invalid - which to me rules out the Oracle CLOBbing function (I have tried several). If I double the CLOB field in the Oracle Query it is always invalid on one row only regardless of sort order, but not related to file size. Any ideas appreciated! Geoff.