As JP and ME stated, overcharge and ground connection would be a great start but another thought on this subject would be checking the intake manifold for crack at #1 cylinder. As unit get warmer the crack will open up and allow unit to surge on top end and run somewhat poor on idle. Also check LPG assist injector for proper operation. A quick removal inspection and check is simple.

Stephen;
Jp has given you a good place to start from. There are three places were grounds are located on this model. Under the left side step plate cover you will find a bolted ground, on the left inside below and behind on the vaporizer - regulator side is another bolted ground connections and on the fuel lock off is the third ground connection. One other item you might look into is corrosion at fuse box. Did you, when you were switching parts, switch the injector driver? This is the small square black box mounted next to the ECM on the left inside. This is only used on Platium I series (LO1/LO2).

stephen does this surging happen while driving? or all the time even sitting still?
actually after i posted the above question i ran across this tidbit of info
check your charging from the alternator
cat had a surging problem on thier nissan K21&25 engines caused by the alternator overcharging and sending too much voltage to the electrical components in the fuel system.
It should not be charging anywhere outside this range from 13.8v to 14.9v. If its charging over this then check the chassis and engine block ground... make sure you have good ground points there. If all is ok there and you still have an overcharging condition then they recommend having the alternator repaired.

the black boxs are ecm and vcm