If problem comes late in the day, when fuel level in tank gets low, it could be the transfer (lift) pump. Remans are reasonable, but pump can be kitted cheaply by replacing the three one-way check valves within the pump housing. Crack loose lines, then remove pump mounting bolts, move pump away from injector pump housing, then finish removing fuel lines. This will keep you from getting a shot of fuel in the face when pushrod jumps. (from experience) If machine falls flat on face after a high demand fuel situation, long hard uphill, or just extended heavy load, check for restriction in your fuel tank/ lines leading to your first filter or water separator. I had a sludge/slime buildup in the bottom of my tank near the fuel shutoff. Popped the line, opened the valve, and the fuel just dribbled out. After cleaning, I think it would freeflow over a quart per minute. Another place to look would be fuel cap venting. Could be one of these, may not be any of these, but they may be things you haven't tried but might want to.
smoothoperator...free advice is worth every penny