Simple things first...
1. When engine starts acting up, rapidly pump the primer bulb. If that smooths it out, suspect your low pressure fuel pump.
2. While engine is running and cover is off, look at the fuel in the on/board water separator and the low pressure fuel filter bowl. They should be about 1/2 full.
3. When engine running, feel primer bulb. If partially collapsed you have a blockage in your fuel tank vent or on the pickup tube in the tank.
4. Inspect the fuel line from the primer bulb to the onboard F/W separator looking for kinks or cracks. Especially where it goes into the big black grommet as it enters the engine compartment.
5. VST - nothing came out? Activate key switch momentarily to charge high pressure fuel pump. Then turn out screw while drain tube is in a clear bottle. If nothing comes out, blow into the drain tube. If still nothing, carefully suck on it or use a battery filler bulb to do so. You should get up over a pint of fuel and possibly water and gunk in the jar. Let it settle out in a clear jar for 20 minutes or so. If water and/or gunk, your low pressure fuel filter has failed. Replace it.
6. If nothing above helps, check fuel pressure at the fuel pressure relief bolt that sits on top of the cover to the HP fuel filter. You will need a 6 mm adapter for your fuel pressure gauge. Spec is 41 - 48 PSI at 600 rpm (idle) with vacuum hose on the fuel pressure regulator removed and clamped. (You need to remove the breather on top of the engine to get to the pressure regulator and vacuum tube. Without doing that, fuel pressure should read between about 37 to 41 psi.)
Let us know what you find and we'll go deeper if necessary.