Mk3 Kuga running issues

[Craig.s]

I recently bought a Mk3 Kuga, the 2.0l diesel. It was running fine for a few weeks, then all of a sudden, when we went to start it, it would start, run very poorly for a few seconds then die. It did this for a day or 2, then all of a sudden it started running smoothly again. Ran fine for a week, then did exactly the same thing again. On the 2 occasions it wouldn't start, we had just filled the car up, I only use Shell fuel so would be surprised if it was rubbish fuel. I have noticed that the oil level is dropping week by week also, so it's obviously burning oil for some reason. The car had a diagnostic ran on it and it was showing that the engine oil level was too high, and also, that it was too low. Has anyone got any ideas what could be causing these issues?