define an integer variable appended with px

basically the script below modifies some CSS to scroll down a div menu, the problem is it doesnt seem to scroll down in pixels ( if i change for example "(<37)" to "(<36)" the layer actually repositions by roughly 10pixels.

var i=-150;
var intHide;

function show()
{
if (i<37)
      {
      i=i+4;
      document.all("foo").style.top=i-1;
      }
}

function showmenu()
{
clearInterval(intHide);
intShow=setInterval("show()",1);
}

function hide()
{
if (i>-150)
      {
      i=i-4;
      document.all("foo").style.top=i;
      }
}

function hidemenu()
{
clearInterval(intShow);
intHide=setInterval("hide()",1);
}