Reading from Arduino Serial seems to be very unstable - c++

Question

I have the following simple code to read commands sent from the computer to an Arduino Mega board:
void get_command()
{
char received;
int index = 0;
while (Serial.available() > 0) {
if (index < BUFFER_SIZE -1) {
received = Serial.read();
if (received == '\n') {
buffer[index] = '\0';
parse_command(buffer);
} else {
buffer[index++] = received;
}
} else {
Serial.println('666'); // buffer overflow
}
}
}
The commands are like A123 B123 C123 D123\n
Basically a sequence of space separated instructions, were each instruction starts by a letter and follows by a number. A command ends with a "\n".
Reading this seems to be very unstable. Sometimes I get the command perfectly, sometimes I miss the first letter, sometimes a pair of them, sometimes it starts by the number,...
I am sending the commands through the serial monitor, set to newline.
Before having this code, I'd simply check for the size using Serial.available, then Id wait a second to fill the buffer, and then I'd copy the buffer to a char*. This worked flawlessly. I am doing now this loop waiting for a \n, which seems more elegant, but it is being very unstable.

Answer 1

Code works by simply adding a delay(100) after the first Serial.available()
This is the code:
void get_command()
{
char received;
if (Serial.available() > 0) {
delay(100); // let the buffer fill up
int index = 0;
while (Serial.available() > 0) {
if (index < BUFFER_SIZE -1) {
received = Serial.read();
if (received != '\n') {
buffer[index] = received;
index++;
} else {
buffer[index] = '\0';
parse_command(buffer);
}
} else {
Serial.println('666'); // buffer overflow
}
}
}
}