Mitsubishi Meldas 500M RS232 Error P460 TAPE I/O ERROR

Looking for some assistance.
I have a Meldas 500M that i am running DNC programs to using the ONECNC free RS232 software.
I am having issues with an error P460 TAPE I/O ERROR. Reading all the forums suggests this error is a buffer overflow issue. I have set up both the PC and Control using a variety of methods as suggested on this group, also as suggested in the ONECNC set up manual.
The problem is no matter what settings i use, baud rate, parity, increase/decrease block delay etc etc i still get this error. In some cases the machine will run perfectly for thousands of blocks and then try again and will only read a few. It is reading a lot of point to point data. Even whilst running at 100% perfectly if i "slide hold" or slow the feedrate down this error occurs.
My settings are as follows:
1. Handshake - XON/XOFF
2. Baud Rate 9600
DTR/DSR - OFF
Databits - 7
Parity - EVEN
Stop bits - 1
RTS/CTS - OFF
Start at Machine - ON
Timing (Line Delay) - 1msec although increasing or decreasing this number makes it worse.

Could this be a hardware or cable issue? Seems to have been working in the past and only recently has been throwing this error.
Any help or advice is appreciated. Or if you have has this issue how did you overcome it??