When Nissan fb15 losing power in drive and or switches off engine completely when in drive or reverse. Also when when in Park and when you rave the engine sometimes hunts, what is the problem?

Possible coolant temperature sensor problem not placing the engine in phase lock loop.

run the fault codes
indicates a problem idle control sensor

run the fault codes first
could be a problem injector control module

check the water in the radiator when in drive is the motor raved up try raving the motor up in park could be the water pump

Does the starter engage again when you go back to park ?
Shifting from park to drive would cut the circuit from the key switch to the starter. The neutral safety switch prevents you from cranking the engine in drive or reverse.
I would say the key switch is not turning off power to the starter when you go from start to run.

run the fault codes and look for cam/crank position sensors.
faulty or loose connections may cause this fault. If you have more than 1 key in the ignition then it has been known that the mass of swinging keys will switch off the ignition
(current recall on all GMC ignition switches for just this reason)

IDLE TOO LOW OR VACUUM LEAKS IN SYSTEM..

Because an engine revs up freely with no load on it in park and neutral is not a test for loss of power.

good day Sir, try this link owners manual i hope this will help you

I am looking for user manual or service manual for Nissan fb15. Could anyone help me.

Regards
Kunchu