setWatch issue

Hi,
I'm having trouble with a setWatch function. I'm monitoring a pulse that is 8uS wide and I want that to trigger the setWatch which contains and SPI write of 32 bits followed by 2 SPI reads of 32 bits. I'm using the MDBT42Q.
It seems to be triggering randomly and I'm stumped. Suggestions?

watchID = setWatch(function() {

    SPI1.write([READ_PTR, 0x00, 0x00, 0x00], nSEN);

    junk = SPI1.send([READ_FIFO, 0x00, 0x00, 0x00], nSEN);
    ecg = 256*(256*junk[1]+junk[2])+junk[3];

//      console.log(ecg);
  
    junk = SPI1.send([READ_LEAD_STATUS , 0x00, 0x00, 0x00], nSEN);

  readCount++;

}, ADC_RDY, {repeat: true, edge:'falling'});