Espruino with Glediator & LED matrix

Sorry to reopen topic but I think it's the best place. Since my last post here I've built a much bigger RGB LED matrix (15x15) and beginning to run into some serial data issues.
On my 15x15 board I can easily output the first 168 pixels but any more and it all goes wrong. I'm not sure if this is an issue with Glediator, USB2Serial, Espruino reading the serial data or Espruino outputting the SPI data?
I don't believe Espruino is the issue but I'm really not sure what is. The code below does the same as above but outputs the "line" length (the graphic SPI data). For 15x15 it should always be 675 but rarely is..

function onInit() {
  SPI2.setup({baud:3200000, mosi:B15});
  var cmd = "";
  Serial1.setup(1000000, {tx:B6,rx:B7});
  Serial1.on('data', function (data) {
    cmd+=data;
    var idx = cmd.indexOf("\1");
    while (idx>=0) {
      var line = cmd.substr(0,idx);
      cmd = cmd.substr(idx+1);
      idx = cmd.indexOf("\1");
      console.log(line.length);
      SPI2.send4bit(line, 0b0001, 0b0011);
    }
  });
}

And the console returns:

Running onInit()... 
0 
675 
504 
664 
504 
504 
602 
504 
504 
504 
545 
504 
1002 
219 
504 
504 
504 
504 
504 
504 
504 
504 
504 
504 
504 
1076 
147 
504 
504 
833 
390 
504 
504 
504 
725 
1128 
88 
504 
504 
504 
661 
504 
504 
1077 
144 
504 
504 
504 
504 
504 
834 
389 
504 
504 
504 
904 
895 

675 only appears a couple of time while 504 appears many but again there are lots of other seemly random lengths.
Is the problem the serial data that's being sent from Glediator or the way I'm detecting a new frame "\1" ?
Let me know if there is anything I can try to debug this or if the magic length 504 means anything? Thanks