Hi,
Here is my experience, I bought a 20 year old barn find 230sl, partially stripped. i assembled the car, replaced all the filters, fluids, put in new plugs, set the timing etc. I had the tank redone using the renew process. Started the car after five minutes of cranking. The car started fine, idled and sounded great for a 20 year old mothballed engine.
It took me a few weeks to finish most of the assembly and was ready for a test drive. In the interim, I would start the engine hear it idle and get excited. At some point, it died and would not start. I tried everything, finally put another new set of plugs and it fired right up. Finally took it for a drive, less then a mile, I started loosing power, idle dropped, came back home and after idling it died again. It would not start. Put in a new set of plugs, it fired up and sounded great. Took it for a drive, after 3 miles, again, started loosing power , low idle, brought it home, it died and would not start. I put in a new set (fourth time) of plugs , it fired right up. After 7000 miles, I have the same plugs and often do an Italian tune up at 90 plus MPH.
I have no explanation except, the experts in this forum often refer to the importance of fresh plugs. I use NGK's. Whatever, it worked for me. Look at my Ellary posts.
Good Luck,