Take your timing light with you and a can of carb cleaner next time you go for a ride, when it shuts off, hook your timing light up to the coil wire and crank the engine to check for spark. If you have spark but the engine still won't start, spray some carb cleaner into the intake manifold and crank again, if it starts, you have a fuel delivery problem...
If the spark check failed prior to spraying into the intake, then proceed to check CPS, coil and wiring as previously recommended, good luck.