This might not be the only problem, but you define DigiData as an array of 74 bytes.
Code:
DigiData            VAR         BYTE[74]   ' Array for rrecieved RS232 Data
But then later you do an i = 0 to 74 (that's 75 bytes), and after that you do a for i = 1 to 74.

you should use:
Code:
For i = 0 to 73
with that variable define, or define as BYTE[75] so you can use for i = 0 to 74