Sorry for not replying so long but for some reason I was not able to login to the forum...
Riva your suggestion about making the delay longer between sending data and turning off the transmit pin on the Arduino side worked...
So to be able to use all the full 1200 bytes bandwidth that is available using the 9600 baud rate in any data transfer protocols, the data needs 1 second to be transferred successfully (which when i think now is obvious but I didn't even think about it until Riva suggestion).
Thanks Riva for your help and for the real solution to the problem.