I OWN A 1998 Isuzu Trooper WITH 3

I OWN A 1998 ISUZU TROOPER WITH 3.5 L ENG. RECENTLY I SWAPPED ENGINES WITH ONE FROM A SALVAGE YARD (REPUTABLE ONE). THE VEHICLE SEEMS TO ACCELERATE NORMALLY EXCEPT GRUDGINGLY (NO MISFIRE)-SUDDELY IT ACCELERATES LIKE IF IT WAS SHOT OUT OF CANNON. OH, AN INSTANT PRIOR TO THE ACCELERATION, THE CHECK ENGINE LIGHT STARTED FLASHING. THEN, AFTER A FEW MOMENTS AFTER CRUISING, THE LIGHT WOULD RETURN TO STEADY AND THE VEHICLE WILL START STRUGGLING AGAIN. FUEL PRESSURE IS STEADY AT 52PSI. REPLACED FUEL PRESS REGULATOR, HAS NO LEAKS AROUND INTAKE. WHILE IDLING, WE, VE DISCONNECTED A SINGLE COIL ON DRIIVER'S SIDE AND VERY NOTICEABLE MISFIRING RESULTING. SAME RESULT ON ANY OTHER COIL CONNECTOR PULLED ON SAME BANK. SAME RESULT WITH FUEL INJECTOR CONNECTOR. ON PASENGER SIDE, WITH THE SAME PROCESS, THERE'S HARDLY NOTICEABLE DROPO IN RPM.