If you want to test the coil you could try this method:

https://www.bing.com/videos/search?...2E70F6D732&view=detail&FORM=VIRE

If the coil appears to be OK then I would check the High Tension cable from the coil for any breaks or weakness this could be causing an intermittent spark.

Bruce
1964 4/4 Series V Comp (Megan)
1994 +8 (Maurice) tis no more
2013 M3W (Olga)