weve had quite a few problems with the inlet manifold gaskets failing and cracked inlet manifolds on these machines this gives a missfire like the one you mention. Also the other comments on the sparkplugs is correct dont fit any after market plugs into any of the grendia Mitsi trucks they really can be problematic

Thanks everybody got loads of info to help me try and sort it out.
Cheers

there is a service gram on this problem
main issues that cause this kevin has hit on
spark plugs
malfunctioning throttle body (usually from being dirty)
also the fuel lock being stuck (halfopen) can cause this

make sure you are using NGK plugs (recommended OEM plugs)
clean the throttlebody and also the fuel lock with solvent

this should clear it up

Another possibility is the throttle has carbon build up on the butterfly,this is a drive by wire system and has no throttle cable.
Can take the air horn off and wipe the throttle plate and bore off with some carb cleaner on a rag or qtip of some sort,dont need to go overboard with it just a good wipe down
Yet another possibility is if it has tar buildup(in the Regulator) that some got to the injector and is causing issues with the injector sticking a little bit

Any Codes showing in the Display at anytime?
Can try doing a Throttle valve closed learning and see if that helps:

OPERATION PROCEDURE FOR THROTTLE VALVE CLOSED
POSITION LEARNING
1. Turn ignition switch ON → OFF. (And hold it for approximately 5
seconds after turning it OFF.)
2. Make sure that throttle valve operates. (Check for operating
sound.)
3. Upon completion of the above, learning is complete.

Can check your fuel PSI at the Regualtor and see what it is running at,maybe the regualtor has tar build up in it,can drain it: let the truck warm up to operating temperature and then shut the fuel off and let the truck die,on the bottom of the rerualtor is a 12 mm hex plug take this out and let the tar drain then reinstall plug.
To check the fuel PSI(which should run around 4.5 PSI) on top of the regulator is a 17mm hex with a phillips head slot in it,take this out and put a pressure guage in it(a 1/8 NPT will work but dont force it in,just hand snug it) start the truck and see what the pressure is at idle and at full PRM,it shouldnt drop when the hydraulics are operated.

Thanks for the info Kev.
The plugs I put in were OEM, got them from a Mitsi engineer.
Truck has 4500 hrs on it.
Tried that with the coils, they work well at idle and with lots of revs its just the period as you put your foot down for a second or two that it acts itself.
Any more ideas.
Thanks

What plugs did you put in this truck,this has the Nissan K21/K25 engine and they dont run well with aftermarket plugs in them,is BEST to use OEM plugs in this truck. As for the coils,you can unhook them so that only one coil is firing and if the engine stays running at a lopey idle then that coil is good,check all four coils individually,any that wont keep the engine running i would replace,might be safer to just replace all four of them anyways if the plugs were worn,how many hours are on the truck??