Weird starting patterns & surging engine when car is at lights

[beege]

Sometimes my VN turns over normally (mostly when the engine is warm I've noticed), other times it turns over rapidly or it takes over a second. What could be causing this? Also when I am at the lights, my engine surges slightly every few seconds. A couple of months ago I got my CAS replaced and recently my plugs and leads. Before I got my CAS replaced I did a codes check and I also got an error for my oxygen sensor, would this relate to my issues above?

[COTTER]

I had a VN that did the same thing ended up being the stepper motor so i gave it a good clean and the problem was fixed.