Nissan Repair: 1993 Nissan Pathfinder runs like on 3 cyl(6cyl), nissan pathfinder, vacuum leak

Question
my Pathfinder runs or chugs as though its hitting on 3cyl.,yet it
will rev up when not under way.  it starts easily.

Answer
First you need to check if there is a major vacuum leak and if the ignition timing and wiring firing order are correct and let me know. Also, let me know if any work was done on the motor prior to this problem. It is important for me to know if anything was replaced altered or repaired. Thanks.