In the last episode (Aug 21), Peter Bortas @ Pike developers forum said:
sync exists on a lot of systems, but not all. That sync helps suggests that the OS i broken. the kernel disk layer should keep the view of the disk consistent ragardless of what has hit real media and what is still in memory.
If it's always the same file at fault, it might be interesting to print the results of ls -l on the file just before and after the tar run (or modify gtar to print the two values at part of the error message).