There is no way to randomise standard actions once defined and running. But there is a workaround you can use to achieve desired effect using [customActionWithDuration].1
SKAction* randomPositionAction = [SKAction customActionWithDuration:0 actionBlock:^(SKNode *node,CGFloat elapsedTime){
int randomY = arc4random_uniform(121);
//Set position instead of running action with duration 0
[node setPosition:CGPointMake((STAGESIZE.width+(self.size.width/2)),randomY)];
}];
RandomY is randomised every time action is run and position is set according to that.