function startCalc(){
  interval = setInterval("calc()",1);
}
function calc(){
  one = document.autoSumForm.firstBox.value;
  two = document.autoSumForm.secondBox.value; 
  document.autoSumForm.thirdBox.value = (one * 600) + (two * 200);
}
function stopCalc(){
  clearInterval(interval);
}