E91D has Strange idling and coughing issue

[Ritchie026]

I have a 2009 E91 318D with 75k. My problem is that my car has an intermittent idling issue but the Rev counter doesn’t move and it has now started to cough (intermittently) as if there’s either not enough air or fuel getting through the engine. I replaced my spark plugs (all but no4) a few months ago and this seemed to cure the idling issue but over the past few weeks it came back and then the coughing started. Fuel filters been replaced and the cars been serviced recently too. Any ideas what could be going on?