It is your code from the link I posted in the question Nick -
#include <SPI.h>
#include <avr/pgmspace.h>
#include "font.h"
// define max7219 registers
const byte MAX7219_REG_NOOP = 0x0;
const byte MAX7219_REG_DIGIT0 = 0x1;
const byte MAX7219_REG_DIGIT1 = 0x2;
const byte MAX7219_REG_DIGIT2 = 0x3;
const byte MAX7219_REG_DIGIT3 = 0x4;
const byte MAX7219_REG_DIGIT4 = 0x5;
const byte MAX7219_REG_DIGIT5 = 0x6;
const byte MAX7219_REG_DIGIT6 = 0x7;
const byte MAX7219_REG_DIGIT7 = 0x8;
const byte MAX7219_REG_DECODEMODE = 0x9;
const byte MAX7219_REG_INTENSITY = 0xA;
const byte MAX7219_REG_SCANLIMIT = 0xB;
const byte MAX7219_REG_SHUTDOWN = 0xC;
const byte MAX7219_REG_DISPLAYTEST = 0xF;
void sendByte (const byte reg, const byte data)
{
digitalWrite (SS, LOW);
SPI.transfer (reg);
SPI.transfer (data);
digitalWrite (SS, HIGH);
} // end of sendByte
void letter (const byte c)
{
for (byte col = 0; col < 8; col++)
sendByte (col + 1, pgm_read_byte (&cp437_font [c] [col]));
} // end of letter
void showString (const char * s, const unsigned long time)
{
char c;
while (c = *s++)
{
letter (c);
delay (time);
letter (' '); // brief gap between letters
delay (10);
}
} // end of showString
void setup () {
SPI.begin ();
sendByte (MAX7219_REG_SCANLIMIT, 7); // show all 8 digits
sendByte (MAX7219_REG_DECODEMODE, 0); // using an led matrix (not digits)
sendByte (MAX7219_REG_DISPLAYTEST, 0); // no display test
// clear display
for (byte col = 0; col < 8; col++)
sendByte (col + 1, 0);
sendByte (MAX7219_REG_INTENSITY, 7); // character intensity: range: 0 to 15
sendByte (MAX7219_REG_SHUTDOWN, 1); // not in shutdown mode (ie. start it up)
} // end of setup
void loop ()
{
showString ("PEDRO ", 600);
} // end of loop
There seems to be no pattern to this problem, some code displays ok and some doesn't. It is definitely a hardware or Arduino sofware issue IMHO (extremely humble 
)