That cap looks like it has more than 100+ hours versus only 5 or so as you
say. Here is what I would do.

1-use a ohm meter and check the resistance of each wire
2-is the coil primary side wiring reversed. when this hsppens the spark
jumps the wrong direction (polarity)
3-it is possible the coil is arcing internally causing the tach to go wacky
4-your coil resistancce being to low may be an indication of an internal
issue.