There is a test that you can do to verify if the coil is opening as it gets heated ,or just replace it, if still original coil.

There is also a pick up coil in the dist that could be experiencing the same problem , in which case the dist has to be removed in order to replace it.

I would suggest trying the ign coil first and if that doesn't help consider the dist pick up coil.
There is a certain procedure to follow before you remove the dist if you have to go that route. If you are familiar with it fine, if not, the procedure is a little lengthy , but if you follow it exactly, you won't even need a timing light to verify all is right.