VB Comms Failure During Long Jobs

Here's the servo handler, probably has no bearing on the problem, but you never know...

void xyzServos() {
  if (zSteps > 0 || (xSteps + y1Steps) < 500) tMult = 2000; // Change acceleration profile to suit Z(B) 
  else tMult = 1000;            // stepper motor which stalls below 3ms
  max1 = max(xSteps, y1Steps);
  max2 = max(y1Steps, zSteps);
  totalSteps = max(max1, max2);
  mx = (double)xSteps/(double)totalSteps;
  my1 = (double)y1Steps/(double)totalSteps;
  my2 = (double)y2Steps/(double)totalSteps;
  mz = (double)zSteps/(double)totalSteps;
  x = mx;
  y1 = my1;
  y2 = my2;
  z = mz;
  laserOn = false;
  if (cut > 0 || laserTime > 0) {
    IOX.write(0x0200, OUTPORT); // Turn lamp & fan ON
    lampState = 0x0200;
  }
  if (cut == 0 && laserTime > 0) digitalWrite(lpower, HIGH);
  for (int i = 0; i < totalSteps; i++) {
    x += mx;
    if (x >= 1 && mx != 0 && xStop == false){ // X-stepper control
      x -= 1.0;
      if (xDir == true) {
        if (xStepIdx == 0) xStepIdx = 4;
        xStepIdx--;
      }
      else {
        xStepIdx++;
        if (xStepIdx > 3) xStepIdx = 0;
      }
      if(twoWire[xStepIdx] & 1<<1){
        digitalWrite(xctrl1,HIGH);
      } 
      else {
        digitalWrite(xctrl1,LOW);
      }
      if(twoWire[xStepIdx] & 1<<0){
        digitalWrite(xctrl2,HIGH);
      } 
      else {
        digitalWrite(xctrl2,LOW);
      }
    }
    y1 += my1;
    if (y1 >= 1 && my1 != 0 && yStop == false){ // Y1(South)-stepper control
      y1 -= 1.0;
      if (yDir == true) {
        if (y1StepIdx == 0) y1StepIdx = 4;
        y1StepIdx--;
      }
      else {
        y1StepIdx++;
        if (y1StepIdx > 3) y1StepIdx = 0;
      }
      if(twoWire[y1StepIdx] & 1<<1){
        digitalWrite(yctrl1,HIGH);
      } 
      else {
        digitalWrite(yctrl1,LOW);
      }
      if(twoWire[y1StepIdx] & 1<<0){
        digitalWrite(yctrl2,HIGH);
      } 
      else {
        digitalWrite(yctrl2,LOW);
      }
    }    
    y2 += my2;
    if (y2 >= 1 && my2 != 0 && yStop == false){ // Y2(North)-stepper control
      y2 -= 1.0;
      if (yDir == true) {
        if (y2StepIdx == 0) y2StepIdx = 4;
        y2StepIdx--;
      }
      else {
        y2StepIdx++;
        if (y2StepIdx > 3) y2StepIdx = 0;
      }
      if(twoWire[y2StepIdx] & 1<<1){
        digitalWrite(y2ctrl1,HIGH);
      } 
      else {
        digitalWrite(y2ctrl1,LOW);
      }
      if(twoWire[y2StepIdx] & 1<<0){
        digitalWrite(y2ctrl2,HIGH);
      } 
      else {
        digitalWrite(y2ctrl2,LOW);
      }
    }   
    z += mz;
    if (z >= 1 && mz != 0){ // Z(B)-stepper control
      z -= 1.0;
      if (zDir == true) {
        if (zStepIdx == 0) zStepIdx = 4;
        zStepIdx--;
      }
      else {
        zStepIdx++;
        if (zStepIdx > 3) zStepIdx = 0;
      }
      IOX.write(fullWaveB[zStepIdx] + lampState, OUTPORT);
    }
    if (cut > 0) { // Put lasing code here..
      bitIndex = (i - (offset1 + space)) % (space + cut + space + gap - 1); // - 1 makes interval agree with vb HorizSteps value!
      if (bitIndex == 0 && i < (totalSteps - offset2)) laserOn = true;
      if (bitIndex == cut) laserOn = false;
      if (laserOn == true) {
        digitalWrite(lpower, HIGH); //rem-out while testing
        delay(laserTime);
      }
      else {
        digitalWrite(lpower, LOW);
        delay(3); // fastest stable transit to next cutting point
      }
    }
    else {
      if (i < totalSteps / 2)
        iTimeIdx = i;
      else
        iTimeIdx = totalSteps - i;
      if (iTimeIdx > 180) iTimeIdx = 180;
      MsDelay = (tMult*(3-(sin((270+iTimeIdx)*PI/180)))) - 1000;
      if (laserTime == 0) delayMicroseconds(MsDelay);
      else delay(laserTime);
    }
    if (digitalRead(xsensors) == HIGH && i > 25) xStop = true;
    if (digitalRead(ysensors) == HIGH && i > 25) yStop = true;
    if (xStop == true || yStop == true) digitalWrite(lpower, LOW); // cut laser power if any limit reached
    if (xStop == true && yStop == true) i = totalSteps;            // bomb-out of loop if both limits reached
  }
  digitalWrite(lpower, LOW);
  //IOX.write(0x0000, OUTPORT); // Turn lamp & fan OFF
}