The hunting ("a bit up and down instead of constant") could be caused by an air leak on the inlet (after the throttle body, before the manifold meets the head). This could affect the emissions....!

With the engine ticking over get yer head under the bonnet and listen for a hissing noise, if you think you hear one spray some wd40 around where you think the noise might be and see if this affects how the engines ticking over.

Just an idea, hope it helps!