Hi, Persy. I've had the same problem on my NP-30g. I've been told that it can be related to the low oil shut-off and/or another automatic shut-off which I cannot recall as I'm typing this to you.
With that said, if you go under the rig and remove the oil filter, you'll find tucked up inside of the housing the two wires that serve these shut-offs. Bearing in mind that the shut-offs are there for safety purposes, some people have successfully cut these wires (you need to cap them off), and solved the symptoms you describe.
Also, if you want to take Dan's advice (he's very knowledgeable) and check the fuel filter, all you need to do is remove it and see if you can blow a good strong flow of air through it with minimal effort. If you can NOT, then it's probably clogged.
If that's your problem, you're very lucky!