Sprite moving with no input

Hello,
I've been working with Unity off and on for a while, but mostly just dabbling while I try out different engines to see which one I like best. Now I have a solid concept for a game I want to create and I've decided to use Unity for it. To start, I made a small scene to test out some of the mechanics I want to include in the game and I have this one problem that is vexing me to the point of exasperation. When I have my character sprite climbing a ladder, he continues moving in the same direction as when I released the arrow button, albeit at a much slower rate. Does anyone have any idea what might be causing this? The relevant code snippet is included below and I also want to point out that the Debug.Log() call reveals that moveUp is in fact 0 when no buttons are being pressed. Thanks.

Code (csharp):

1.  
2.         moveUp = Input.GetAxis ("Vertical");
3.         Debug.Log (moveUp);
4.        
5.         if (Physics.Raycast (transform.position, new Vector3(0, 0, 1), out hit, rayLength)) {
6.             if (hit.collider.gameObject.tag == "ladder")
7.                 climbing = true;
8.             else
9.                 climbing = false;
10.         }
11.  
12.         if (climbing  Mathf.Abs (moveUp) > 0) {
13.             rigidbody.useGravity = false;
14.             rigidbody.velocity = new Vector3(0, moveUp * maxClimbSpeed, 0);
15.         }
16.         else if (!climbing)
17.             rigidbody.useGravity = true;
18.  

 

change this
rigidbody.velocity = new Vector3(0, moveUp * maxClimbSpeed, 0);

to this

transform.Translate(Vector3.up * moveUp * maxClimbSpeed);

See if its any better

 

That makes it move ridiculously fast where before it moved at my intended speed. It did fix the drifting sprite, issue, so it's a starting point. I just need to dial in moveSpeed to get it where I want it. Thanks.

EDIT: I also noticed that if I don't get my character sprite just right on the ground at the bottom of the ladder, he'll tunnel through the floor, hang out for a second and then start rising without responding to any of the controls I use. I believe this has something to do with an issue I read about recently where Box2D doesn't play well with the non-physics functions.