It sounds as though you have determined that when it does not start, it is not getting any spark, right?

If it is not getting spark. You may need to get a voltmeter and start working your way through the ignition system until you find the problem. I'm not familiar with your particular system, but somewhere you have to have a magnetic sensor that initates a low voltage pulse as the engine turns. That pulse goes to an electronic controller that tweeks the timing and turns the weak pulse to a strong on/off DC current that goes to the primary side of the coil. Every time the DC current shuts off, the coil fires a spark that goes to the distributor cap, through the rotor and down a spark plug wire.

You'd better get a manual while you are buying that voltmeter rather than trying to work from my vague description.

Most likely scenaro from my VERY limited experience: The electronics that (I think) in your model are in the distributor. Junk yard distributors are not expensive and can be replaced in minutes.