I am assuming that when it does not fire it is not getting fuel,
and that you are running the engine at 3200 - 3400 RPM,
if it was getting fuel you should smell the diesel coming out
of the stack. There is no 12 volt blower motor here, but it is
very important for combustion that the engine is running at
proper RPM. The 4305-2T uses a belt driven blower wheel and
fuel pump, we can look at fuel solenoid and ignitor next.
If it was the ignitor (very unlikely to have intermittant problem)
it would load up with fuel & then ignite with a roar like a freight
train. You might take the cover off of the pressure switch and see
if the plunger is engaging the microswitch every time you trigger the gun.
I am also asuming you are getting good pressure through the gun.
(If the pressure is too low it would not activate pressure switch.
Call me Monday morning if you need further assistance.