Nissan pathfinder sounds/feels like a temporary shutdown of injectors when rev from first to second and between gears while driving with increased speed. what could this be.

Possibly the wire across the venture of the MAF is dirty. It takes a special spray to clean it. Could also be a dirty or bad fuel injector causing it to do this. Have it checked with a code reader to find which is the dead cylinder if any. If is #5 cylinder dead reading will be P0305 and so on. It was a bad injector on mine.

what is a valve board?? check plugs,wires,coils

Are you in gear? What are the idle revs showing on the tacho? Have a scan done on the computer via the OBD2 port and see if any trouble codes identify problems eg spark plug, coils, etc.

The gears inside your transmission are going bad. You are going to need to replace your transmission.

Engine system needs inspection on case of backfire:-----

Check out related help links to troubleshoot the vehicle backfire issues:-----

Feels like backfire from engine when decelerating?

http://schematicsdiagram.blogspot.in/2012/02/feels-like-backfire-from-engine-when.html

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Jeep Backfires?

http://technoanswers.blogspot.in/2012/02/jeep-backfires.html

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Nissan Pathfinder stalls and bucks on Acceleration?

http://technoanswers.blogspot.in/2012/02/nissan-pathfinder-stalls-and-bucks-on.html

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Nissan Pathfinder Backfires?

http://technoanswers.blogspot.in/2012/02/nissan-pathfinder-backfires.html
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These will help.
Thanks.

sounds like it is not shifting out of 3rd gear. count the shifts.

Nissan Terrano's have an Electronic Computer that controls the gearbox. It is hidden behind the left hand rear Speaker box, right hard up against the rear seat back rest. This system controls all functions regarding the gearbox and it's rev rates. If your vehicle is shifting back on long drives then it is possible that the computer sees any slight increase in revs as a desire to "Kickback" the gearbox to a lower gearing, hence it dropping out of overdrive. Unfortunately a gearbox specialist with a computer "Plug-in" is needed to fix this, but it is fixable. This problem should not be too expensive in comparison with a new gearbox.

Has your fuel economy dropped? It may not have, what it sounds like to me is a low rpm miss. You could have a dirty injector or injectors, hard to tell unless hooked up to engine analyzer

sounds like a trasmission computer box is bad or the transmission module is out.

does this happen between gear changes ?