[77newell]

If it runs longer after changing filters then it very well could be gunk in the tank.

You didn't mention if it restarts the same after it quits. Does it just suddenly quit or does it gradually die. All a diesel needs is air, compression and fuel. My thoughts and that of the other posters is that fuel is going away. A restriction in the fuel supply would tend to make the engine more gradually die and perhaps sputter on the way there. If the safety shutdowns are triggering it I would expect the shutdown to be sudden and complete. It may help to extend the generator so you can see the shutdown solenoid and see if it moves the shutdown control rod at the instant the engine quits.

Did the hard starting begin at the same time as the sudden shutdown?

All the questions when answered help lead to the likely source of the problem.