I don't have this hardware, so I haven't experienced the problem.  However,
from other serial port work, I have found that problems can occur with
hardware low control when serial port cache is enabled.  When the peripheral
sets hardware flow control to stop the PC sending, the PC doesn't stop until
the port cache is empty, which results in a few more bytes being sent.  If
the peripheral has no ability to receive any more data after telling the PC
to stop, these bytes are lost.

Try disabling the COM port cache?
Ed Todd <edtodd@sni.net>
http://www.sni.net/cedardell