SKSpriteNode vibrating when physics body is attached and impulse is applied

Question

I've noticed something very odd with my subclassed SKSpriteNode with physicsBody attached and responding to impulses.

It has a SKPhysicsBody attached, but when applying impulses to the sprite it vibrates during movement. It's highly annoying and looks awful.

What could be causing this?

Here's the setup code:

self.texture = [[SKTextureAtlas atlasNamed:@"karl"]textureNamed:@"karlidle-1"];

    self.size = CGSizeMake(69.0, 93.0);
    self.userInteractionEnabled = NO;
    self.name = @"karl";

    self.physicsBody = [SKPhysicsBody bodyWithRectangleOfSize:CGSizeMake(69.0, 93.0)];
    self.physicsBody.dynamic = YES;
    self.physicsBody.allowsRotation = NO;
    self.physicsBody.affectedByGravity = YES;
    self.physicsBody.usesPreciseCollisionDetection = YES;
    self.physicsBody.mass = 1.0;
    self.physicsBody.linearDamping = 0.0;
    self.physicsBody.angularDamping = 0.0;
    self.physicsBody.restitution = 0.3;

    self.physicsBody.categoryBitMask = karlCategory;
    self.physicsBody.collisionBitMask = obstaclesCategory|floorCategory|wallCategory;
    self.physicsBody.contactTestBitMask = obstaclesCategory|floorCategory|wallCategory;

and applying the impulse:

 if (self.physicsBody.velocity.dy >= kKarlMaxVelocity) {

        self.physicsBody.velocity = CGVectorMake(0, kKarlMaxVelocity);
    }
    else if (self.physicsBody.velocity.dy <= kKarlLowVelocityBoostThreshold){

        self.physicsBody.velocity = CGVectorMake(0, kKarlVelocityImpulseIncrement*kKarlVelocityBoost);
    }
    else{

        [self.physicsBody applyImpulse:CGVectorMake(0, kKarlVelocityImpulseIncrement)];
        //[self.physicsBody applyForce:CGVectorMake(0, 3000) atPoint:self.frame.origin];
        //self.physicsBody.velocity = CGVectorMake([self returnNormalisedXVelocity], self.physicsBody.velocity.dy);
    }

One thing to note is I clamp the velocity so it doesn't go nuts, but turning this off doesn't seem to fix the issue.