Lerping 3d rotation (away3d)

I'm creating a 3d game using away3d and awayphysics.

I've created a rotation formula that will rotate my model with a "smooth factor".

private var chRotation:Number = 0;

public override function update(delta:uint):void
{
    if(target){
        var smooth:Number = 0.95;
        var tp:Vector3D  = target.ghostObject.position;
        var cp:Vector3D  = entity.ghostObject.position;
        var targetAngle:Number = -((180 / Math.PI) * Math.atan2(cp.z - tp.z, cp.x - tp.x));

        if(oldTgRotation - targetAngle != 0){
            if((oldTgRotation - targetAngle) > 300){
                chRotation = -180;
            }else if((oldTgRotation - targetAngle) < -300){
                chRotation = 180;
            }
        }

        chRotation += (targetAngle + (chRotation - targetAngle) * (smooth - (delta / 800))) - chRotation;

        entity.ghostObject.rotation = new Vector3D(0, chRotation, 0);
        oldTgRotation = targetAngle;
    }
}

this works partly, it works until the mesh rotates from -180 to 180 cus the code will then rotate the mesh backwards, so: -180 -90 0 90 180

It should go from -180 to 180 forward. but how?

Edit: I've added kind of a solution but this still isn't perfect:

if(oldTgRotation - targetAngle != 0){
    if((oldTgRotation - targetAngle) > 300){
        chRotation = -180;
    }else if((oldTgRotation - targetAngle) < -300){
        chRotation = 180;
    }
}

Ok, so to fix this problem I've added a boolean switch:

private var chRotation:Number = 0;
private var switchf:Boolean = false;
private var switchb:Boolean = false;

public override function update(delta:uint):void
{
    if(target){
        var smooth:Number = 0.95;
        var tp:Vector3D  = target.ghostObject.position;
        var cp:Vector3D  = entity.ghostObject.position;
        var targetAngle:Number = -((180 / Math.PI) * Math.atan2(cp.z - tp.z, cp.x - tp.x));

        if(oldTgRotation - targetAngle != 0){
            if((oldTgRotation - targetAngle) > 300){
                switchf = true;
            }else if((oldTgRotation - targetAngle) < -300){
                switchb = true;
            }
        }

        if(switchf){
            if(chRotation >= 177){
                switchf = false;
                chRotation = -180;
            }else{
                targetAngle = 190;
            }
        }
        if(switchb){
            if(chRotation <= -177){
                switchb = false;
                chRotation = 180;
            }else{
                targetAngle = -190;
            }
        }

        chRotation += (targetAngle + (chRotation - targetAngle) * (smooth - (delta / 800))) - chRotation;

        entity.ghostObject.rotation = new Vector3D(0, chRotation, 0);
        oldTgRotation = targetAngle;
    }
}