Thanks for the info', Roman... Yeah, the displays are pretty small.

The Datasheet for the controller IC specifies a 4.0-MHz serial clock so I was going to start there.

 

I just checked my notes, I was orig running fosc/4 which was top SPI speed of 10MHz, whcih worked ok but would give a pixel error after a couple hours of screen writing.

I then changed it to fosc/16 which is SPI at 2.5MHz and no problems after that.

My notes also say the top speed is officially 4MHz so i probably looked it up in the display datasheet after that!