Will not start sometimes, sometimes fired right up

When the problem is ongoing, does it crank okay? Is there a security/theft lamp on the dash that is on or flashing? If the vehicle cranks okay when problem is ongoing and no security lamp on, the security system wouldn't be my first thought.
If it cranks good but will not start, check the basics. Have a helper crank it while you check for spark at the plugs. If spark everywhere, use a gauge and check proper fuel pressure and fuel injector pulse. Have to test when problem is ongoing. Any applicable trouble codes? To check codes, at the least, you need an obd2 code reader.