Matlab - Arduino Due - Mac OS X problem

Hi, i've been trying to implement a stable serial connection between matlab and arduino due. What i have to do is pretty simple, i have a string of 8 values (0 or 1), i want to send it to arduino and process it in a loop to turn on and off a digital output consequently. I have a couple of problems: 1 - sometimes Matlab gives me the error (only sometimes, no clue why) Open failed: Port: /dev/tty.usbmodemfa131 is not available (port name is an example, i always check for the name and modify it) while the port is connected as i can see from terminal or trough other serial communication tools (arduino IDE) (and yes i closed all the other tools before) 2 - arduino is not receiving the right values, in fact if i monitor the received string i have some 5 and 3 values, no clue why. Can somebody help me plese? The matlab code is:

clc

answer=1; % this is where we'll store the user's answer
arduino=serial('/dev/tty.usbmodemfa131','BaudRate',9600) % create serial communication object on port tty.usbmodemfa131

fopen(arduino); % initiate arduino communication
v1 = [0 1 0 1 0 1 0 1];

while answer < 8
    fprintf(arduino,'%s',char(v1(answer))); % send answer variable content to arduino
    answer = answer+1;
    pause(0.5);
end

fclose(arduino); % end communication with arduino

and the arduino code is

int matlabData [8];
int i=0;

void setup() 
{
  pinMode(13,OUTPUT);
  pinMode(12,OUTPUT);
  Serial.begin(9600);
}

void loop() 
{
   for(i=0;i<7;i++){
     if(Serial.available()>0) // if there is data to read
        {
        matlabData[i]=Serial.read(); // read data
        }
    }
   i=0;
   for(i=0;i<7;i++){
     digitalWrite(13,matlabData[i]);
     digitalWrite(12,matlabData[i]);
     delay(500);
   }
}

Can somebody help me please? thank you