missing bytes sending binary files on FTP

Hi Everyone!
I try to send files with FTP, using WIPSOFT V2.02. Hardware is a FASTRACK 1306B controlled by a PIC based panel,
handling CTS-RTS lines.
There are no problems if the file contains only ASCII characters, but if I send binary file and the file
contains ETX characters (DLE+ETX) there are some missing bytes after ETX.(10-120 bytes, not after every ETX,
only about three times at 32KB binary file).
These errors only occur if the file contains ETX characters AND the modem buffer becomes full.
No errors occur at ASCII files, or a binary file without ETX characters EVEN IF MODEM BUFFER BECAMES FULL.
Note: If I send the bytes slower, and the MODEM buffer does not become full, there are no errors.

Commands are from PIC to modem:

AT+WIPCREATE=4,1,“ftp server”,21,“username”,“password”

AT+WIPOPT=4,1,2,40,1 rem: switch to binary mode (?)

< data transfer if ETX then DLE+ETX >
ETX - end of data transfer
What is the problem?
Thanks: Medveczky Endre