Sure, just use "Sprite.Animationframe+1" when setting the frame.
I just realized that's what you really wanted, sorry for the misunderstanding! <img src="smileys/smiley17.gif" border="0" align="middle" />
Edit: Oh, and if you want to do either +1 or -1 randomly, you can use the choose expression "Sprite.Animationframe+int(choose(-1, 1))"