working in firefox but not in chrome

working in firefox but not in chrome

the following line of code is NOT working in chrome/safari as it is supposed 2
its working perfectly in firefox and Opera however:

$('#box8').delay(1100).animate({"left": "+=300px"}, 5000);

in firefox/opera the div box slides from its place to the right 300px
but in chrome/safari it suddenly jumps to the right and then slides to the left 300px leaving it half through the page!