Hi folks. My series 5 Titan has developed an issue where it will idle at about 950 revs as opposed to 700 ish. It happens at different times and I can't see a pattern to it. I.e. it's not just when its cold or only when the lights are on or anything like that. it seems to happen at random times and them will revert back to normal randomly too. It does it when the AC is off so it's not that either. It also seems to be running rough when trying to hold a steady speed as well and seems really laggy and lumpy. It has recently been serviced and they said that they couldn't find anything wrong with it.

Does anyone have any idea what the issue could be????

My Golf does this all the time - I was told by the dealer (who I don't trust by the way) that it is the Diesel Particulate Filter regenerating itself (I think that means burning fuel to clear it).
Do you do a lot of short trips? If so this could be the answer....

Normally long journeys to be honest

Have you checked your oil level as thanks to the egr a failed regen will dump into the engine block and raise the oil level