I then checked the spark plugs and engine compression to exclude engine wear and found this:
All spark plugs showed only minor deposits evenly distributed (maybe running a bit on the lean side), and compression values were between 12,5 and 13,5 bar on all cylinders.
I call that a good result for an original engine that has covered 225k km (140k mls). The keen observer might also spot that I changed the valve cover for a freshly powder coated BG GT item:
I now suspect the distributor as the root cause for the intermittent failures - this will be addressed next.