Sounds like a (vague) circuit in the board itself. I too have experience on that. For instance like a display sometimes running great and sometimes "echo-ing it's neigbour. The board was factory made (tested) and out of a batch that was ok. Still after hours and hours I just took a knife and scraped the circuit around a suspicious pin and bingo. I already checked with my multimeter before this and no short could be found. Board was ok after this.