When doing a down impulse while on the ground, it causes the impulse to bounce back into the ball which causes the ball to fly up. You can fix by getting rid of the down arrow impulse.
When pressing left or right, a force is applied. When you let go momentum keeps the ball going. You can get rid of momentum by increasing friction of the ball.
If you want to look at better movement, check out the platform demo.
By the way, major update. I've switched from direct sockets to easy sockets. 200-300% performance gain.