first off, I have a mid 70s motobecane nomade. the past weekend i repainted it and in doing so noticed the the seattube bolt (i think this is what its called) was pretty stripped out so i pulled it out and replaced it with a somewhat smaller bolt (all i had at the time). after tightening it back up again with the regreased post installed, it held its height fine for a while. a couple days ago i loosened it up to raise the seat a bit. after retigthening it, it has been continually slipping. today i even wiped the grease off and tightened it as much as i could and nothing. slipped oveer the first small bump. any ideas here?