I had a similar problem mate, I also couldn’t see any arching from the coil. So l check and changed all the normal bits plugs, leads, fuel filter encase it was a bit clogged ect.. and no luck so l got a coil as a last attempt and it sorted it and it only had one or two tiny hairlines in it:) so don’t go by the arching to much, if it is cracked l would advise changing it