Some players "daytrade" popular stocks, which may cause the movements you are seeing. You are not alone selling a stock only to watch it go up the next day, week, etc... it happens to all of us.
The only thing to do is to worry less about the shorter term gains as your port grows, and accept the fact that SOME stocks are just going to go up and down over time...
The more popular stocks (usually stocks opening soon) players know that people will buy or short openers, so they daytrade them, causing them to go or down depending on who's doing what... they buy them, take some small profits, and then sell them. Watch for news on a movie, a star attached, director, etc, that may affect it's price..perhaps a movie is delayed for some reason - causing the price to drop - some players "play the players!"
Also, HSX has never said how their computers calculate stock movements - so if 10 people sell off MOVIE stock at $50, will the price go down? How about if 100 players sell it? Or short it? How many shares of selling or shorting causes a stock to drop 2 points? 5 points? No one knows... and HSX isn't telling.
You'll either have to accept the movement, or decide if you want to daytrade some of these stocks - which many players do, which may cause the movement you see.