First of all, if your primer bulb is partially collapsed when the engine is acting up, that is a dead give away that either the vent to the fuel tank is clogged or the pick-up tube inside the fuel tank is clogged, or the check valve in the primer bulb has collapsed.
Drain the VST and test again.
If problem is still there, hook up a portable tank with new fuel as suggested by William01 above and see if the problem persists. If so, then there is a possibility that your low-pressure fuel pump is failing. Pull it, inspect it and test it. But first check the bowl seals on the onboard fuel/water separator and the low-pressure fuel filter.
If none of that works, hook up a fuel pressure gauge to the pressure relief bolt that sits on top of the high-pressure fuel filter cover. (You will need a 6 mm adaptor.) Pull the vacuum hose off of the fuel pressure regulator and clamp/seal it. Start the engine and let it warm up. At idle (600 RPM +/-) you should read between 42 and 49 PSI. If pressure is too high, then pull the fuel pressure regulator and clean/replace it. If still too high, then you will need to pull the fuel pump and clean the fuel screen on the bottom of the plastic casing that the fuel pump sits in. That's a real PITA! If you do this, have a replacement plastic cradle for the fuel pump on hand, because there is at least a 50-50 chance you will break the old one getting it out. Don's ask why I know that.
If none of that works, come back at us.