ActionScript 2.0 :: Alpha Of MovieClip To Rise And Low At Set Amount

I am still new to actionscript. What I want to do is set the alpha of a movieclip to rise and then lower at a set amount. I can do it so it set it at random but not the way I want it, e.g., clip starts at 20%alpah, and rised by 1 until it at 50% and then reduces by 1 until back to 20% over and over.

Second frame :
i = i+1;
setProperty ("/grid", _alpha, +i);
result = getProperty("/grid", _alpha);
if (result = 50) {
setProperty ("/grid", _alpha, 50-i);
}

Third frame.
gotoAndPlay (2);