Are you saying, when the problem occurs, the vehicle cranks good but won't start? If so, when the problem occurs, do the basic tests. Have somebody crank it, while you check for spark at the plugs. If you have plenty of spark, check fuel pressure. Are there any trouble-codes?
Intermittent problems are a pain, you have to do the testing when the problem occurs. Maybe you will get lucky and a code has set, that might point you in the right direction.