Q: 07 xterra cranks but wont always start. sometimes starts and shuts off immediately. sometimes just cranks and cranks

The best way to diagnose a problem like this is to play with certain components and attempt to replicate the issue. Components that could be causing these symptoms are the fuel pump, fuel filter, a number of solenoids and sensors, faulty ignition coil, or spark plug. You should absolutely scan the vehicle for diagnostic troubleshooting codes. This would give insight to any sensors or electrical connections that have failed. This scan should be performed when you get the vehicle to start and is running. There may also be mechanical issues in the starting system that are less likely to occur, but can cause these issues. A full inspection of the vehicle is needed. I would recommend having a YourMechanic professional technician repair repair and diagnose the reason to why your car is not starting.