I did the same kind of exercise, see on this page (
http://www.aliquotes.com/remonter_suite_envers.html) my go backwards of 3, 7, 11, 13, 17 on more than 2000 iterations, as well as the go backwards of 2005020 on more than 1000 iterations (I was talking about it here #7 :
http://www.mersenneforum.org/showthread.php?t=18641)