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If after having checked and cleaned all the connections in the ignition system it sure looks to me as though the problem lies in the pick up coil. Typical symptoms are intermittent loss of spark as the motor heats up -- return of spark with cooled motor. Repeat of this scenario until final loss of spark.
Not having experienced this in many years I went through all the connections on my '75 B20F distributor powered '69 144S while returning from a track day at Pocono Raceway last fall. Stuck on the shoulder of I-80 it restarted and would drive a short distance before conking out again after going over connections (no doubt working again due to cooling rather than cleaned connections). Luckily it failed a final time while on the downside of a 7 mile grade and I was able to coast into a PA rest area. I found it was possible to sleep sitting upright overnight :-) !! In the morning I retrieved the original points and condenser distributor packed deep in my "spares" box.
I'm not sure why - but in the mid-late '80's while working in an indy Volvo shop I replaced these failed coils on a number of cars -- but haven't (except for mine) since then. Replacing the coil is straight forward - only the retaining snap ring and reluctor can be tricky as the reluctor is positioned with a TINY solid steel pin inserted in a slot formed by a groove in the dist. shaft and the inside of the reluctor - don't lose it. On one car I found that pin to have completely disintegrated by rust. -- Dave