Simple things first...
Make sure that the vent to your fuel tank is open - run a length of 400 lb. mono (or weedwacker mono) down the vent hole into the tank and work it back and forth a bunch of times.
When is the last time you changed the filter on your external fuel/water separator (e.g. Racor)? If more than a year, change it. Drain the external fuel/water separator and see if you get any water.
Squeeze the primer bulb a bunch of times to make sure it gets hard. If not, replace it. (The check-valve inside of the primer bulb will sometimes fail and partially block fuel flow.)
Carefully inspect the area where the fuel line enters the engine compartment through that big black grommet. Corrosion will often build up in there and pinch the fuel line.
Drain your VST and see if you get any water in whatever drains out. If you get any debris, it means that your fuel filters have been compromised at some point.
If none of that helps the situation, then hook up a fuel pressure gauge at the fuel pressure relief bolt on the top of the high-pressure fuel filter cover. You will need a 6 mm adaptor. Pull the vacuum hose from the fuel pressure regulator and clamp it closed. Run the engine and observe fuel pressure. At idle (600 rpm +/-) you should read 42 - 49 psi. If higher than that, you should clean the fuel pressure regulator (there is another fuel screen in there, also.) If lower than that, then you should pull your HP fuel pump and clean or replace the plastic holder that it sits in. There is a fuel screen on the bottom of it.
That's top-of-mind. Other folks on this forum may have additional things to check.
Let us know what you find.
