I had something similar happen before (M43TU) a slight misfire at idle. I replaced the HT leads with a good used set from Valdas. The misfire improved but didn't go away 100% until I changed the plugs.
I had been previously given bosch non-longlife plugs by a Motor factor. When I installed the correct NGK BKR6EQUP plugs it ran perfectly smooth and has done ever since.