OLED 16x2 With Arduino Yun Error

I've been using 16x2 OLED displays on Megas and Unos for a while with Adafruits' CharacterOLED library (OLED_V2). However, am now attempting to use the same displays on a Yun and it's gone back to the old 'ax ax' error where the screen simply (although not always) displays the alpha x-bar characters. Anyone who has used these will know what I'm talking about.

The code is now stripped to the minimum :

#include <Adafruit_CharacterOLED.h>

Adafruit_CharacterOLED lcd(OLED_V2, 13, 10, 12, 5, 4, 3, 2);

void setup() 
  lcd.begin(16, 2);
  lcd.setCursor(0, 0);
  lcd.print("hello world");

void loop() 

Struggling to understand why the code would work fine on a mega/uno but fail on the Yun. Worth mentioning that I'm powering the Yun and LCD from a 5V reg where on the Mega/Uno it would have been from the 5V out. Guessing it's a power-up or timing issue.

Suggestions ?

The most common error that I see here in the forum from people claiming that their stuff runs on a Uno/Mega or even a Leonardo but not on a Yun is that they do not take into account the differences between the former two in general and the Leonardo in particular. The serial port/pins are probably top of the list here...