File Sink Behavior

I’m testing a radio by sending known data through from a file source,
file sinking the data at the end of the Tx/Rx loopback chain. I’m
very specific erratic behavior with the contents of the output file. One
two things happens. First, the output data is perfect. Second, the first
line of the input file is repeated once, and then the data is perfect
out. I’ll represent the behavior below:

Line# InputFile.dat OutputFile.dat w/
1 Hi how are you Hi how are you
2 1234567890 Hi how are you
3 Hi how are you 1234567890
4 1234567890 Hi how are you
5 Hi how are you 1234567890
and so on

When doing BER calculations, this makes the measurement useless, because
looks like constant errors due to the first line repeat.

I’ve tested this with the File Sink Buffer on and off. It occurs in both
situations. There is no recognizable pattern to me, sometimes it happens
and sometimes it doesn’t. Is there a buffer that does not get flushed
somewhere that could cause this? Has anyone seen this behavior before?