3.2 TFT screen flashing

Got a real odd one I can't work out - if I try out the demo UTFT code from Henning Karlsen everything works great. Now I"m trying some simple code to display something on the screen. It works but after a few seconds I get these weird vertical lines quickly flashing across the screen. I'm using the below code - the strange thing is if I remove this line then it works fine!

I'm thinking its got something to do with screen refreshes but I can't figure out what.

  myGLCD.print("testing 1 2 3", 0, 40);
// http://henningkarlsen.com/electronics/library.php?id=34


#include <UTFT.h>

int x=0;
int y=0;
int z=0;

// Declare which fonts we will be using
extern uint8_t SmallFont[];

// Uncomment the next line for Arduino Mega
UTFT myGLCD(ITDB32S,38,39,40,41);   // Remember to change the model parameter to suit your display module!


//--------------------------------------------------------------------------------------------------------------------
//                                                          SETUP
//--------------------------------------------------------------------------------------------------------------------

void setup()
{
  Serial.begin(9600);

    randomSeed(analogRead(0));
  
// Setup the LCD
  myGLCD.InitLCD();
  myGLCD.setFont(SmallFont);
  myGLCD.clrScr();
}


void loop()
{  


  myGLCD.printNumI(x, 0, 10);
  myGLCD.printNumI(y, 0, 20);
  myGLCD.printNumI(z, 0, 30);
  myGLCD.print("testing 1 2 3", 0, 40); 
  x = x + 1; 
  y = y + 2;
  z = z + 3;
 
}

try to change the color of font