One problem is that you are grabbing 14 characters and assuming that is one complete message. If the first character you read is not 0x1_ then the whole message will be wrong. You should check the top half of each byte and throw away any bytes that don't fit the pattern.
void Get_data() {
int n = 0;
while (n < 14) {
if (Serial.available()) {
byte c = Serial.read();
if (c >> 4 == n + 1)
DMM_ARRAY[n++] = c; // Fits the pattern
else
n = 0; // Try again from the beginning
}
}
}