There are several possibilities ;-
1) Open condensor (replace)
2) Dirty points (replace)
3) Intermittant coil (possible but rare)
I would suggest replacing your points and condensor with a retro-fit electronic conversion module. This will eliminate the possibility of dirty points now and forever.
Alternatively, if the coil is mounted OUTSIDE the flywheel, replace the whole coil unit with a later electronic version.
Neither option is expensive.
If you need detailed instructions, let me know.
phil_saunders@bigfoot.com
Phil Saunders
January 2009