Please recommend a dnc program with quick Xon/Xoff response

I'm looking for a DNC program to drip feed a Mitsubishi Meldas 50 based Mori Seiki. This controller has very limited buffer size and most modern DNC programs seem to quit sending too slowly. I'm currently using software handshake, everything is fine once the program gets started, but the first lines are a problem. 8 out of 10 times it will quit with a P460 i/o error. Web-knowledge suggests using a program capable of Xon/Xoff mid-line, most DNC programs have no specs regarding Xon/Xoff performance. Baud rate doesn't seem to make any difference, slower is almost worse than full speed. Normal file transfer works perfectly, it's just drip feed that goes all quirky.

I'll be happy to buy a working solution, doesn't need to be free..

Thanks/Harri