Issue with my character bobbing up and down

I am having a issue with my character bobbing up and down when he walks. I am making a platformer style game, where the physics are similar to a game like Super Mario Brothers. I apply a constant force to the scene so that all objects are pulled towards the ground. Then I apply a constant force to the character to make him walk back and forth. The problem is, when the character touches one of the "seams" in a tilemap collision polygon, he kind of gets caught on it. This makes the character bump up a little when he hits it. If you are going really slow, in some cases it will actually stop him from moving.

I configured the collision material for the character as follows:

datablock fxCollisionMaterialDatablock2D(knightMaterial)
{
	friction = 0.0;
	restitution = 0.0;
	relaxation = 0.3;
	density = 0.01;
	forceScale = 1;
	damping = 0.1;
};

I setup the collision material as follows (I set the desitiy to 0.0 so that the tilemaps don't get effected by the gravity):

datablock fxCollisionMaterialDatablock2D(tileMaterial)
{
	friction = 0.0;
	restitution = 0.0;
	relaxation = 0.0;
	density = 0.0;
	forceScale = 1;
	damping = 0.1;
};

To create gravity for the scene I did the following (I set the gravitic field to false, because it didn't do anything, no matter what value I set it to):

t2dSceneGraph.setConstantForce("0 10000", false);

To make sure my character wouldn't rotate around, I set his maximum angular velocity as follows:

%this.sceneObject.setMaxAngularVelocity(0);

Finally to move the character I did something like the following:

%this.sceneObject.setConstantForce("5000 0");

Anyone know how to tweak the physics to make it so my character won't bob up and down when walking?