The angle of the yoyo should only be a little off from the plane of the strings.
The more you touch the sides of the yoyo, the faster it will flop, but the more spin it will lose, and when you lose spin it looses stability.
Try going slower and varying the tension and angles until you get a feel for how to control the rotation without losing spin.
Also, as stated before, string should not touch yoyo.
Andre is really good at this if you watch his videos you can see how well he controls it.