Cylinder 6 keeps misfiring

Hi everyone. I need your help
After replacing a Leakey antifreeze reservoir with a week or two my 740i e38 started to misfire on cylinder 6 I went ahead and changed the spark plug and misfiring stopped so I changed the other spark plugs too a month later the same cylinder started to misfire again going on and off and ended to be off line most of the time , please help. Thank you.

Bad coil pack; bad lifter or oil coming down the inlet valve guide. This ignores damaged wires, bad earths etc. How are compression numbers?

Hi
The plug comes out dry would this rule out the oil leak, did not check the compression but the plug worked fine for a month after replacing this might also rule out the bad earthing too, is there other checks that I can do befor going to compress test, thanks.

Lifter becomming defective or coil pack. Change the coil pack with one of the others and see if the fault moves.

Hi
Cylinder 6 has the earthing wire on it so I tightened it since it was loose a little bit everything is fine till now I will give it couple of days and confirm thanks for the help.

Now undo it and clean up the earth point and all the terminals with emery or wet and dry, refit. The do the same the other side. Then do every other earth point and connector plug you can find. That should prevent further, similar, problems.

Thanks I will do that. Car is stable now.