for anyone looking at this post...
the problem was indeed hardware. but it was not the pc hardware.
i was using another PCB that has a 3.3V MCU on it and THAT was the issue.
So although the HC-05 was powered with 5V, the Rx & Tx pins were powered at 3.2V which would appear is insufficient for the correct Tx or Rx of 'SOME' values.
eg Sending the bytes 0h, 5h, 9h from the board were received at the pc as 80h, 85h, 89h (but in a random manner).
Conversely when sending the bytes 0h,5h,9h, from the pc they were received at the board as 80h, 85h, 89h.
Conclusion...use a 5V MCU with a HC-05 (or boost the voltages).