Reading back through this thread I have a sneaking suspicion that something simple and perhaps too obvious has been missed. If I'm wrong then apologies to all...
If the engine isn't running then the fuel pump only runs for about 20 seconds after the ignition is switched on before the electricity supply to it is cut off so the pressure will drop off quite quickly, mainly by bleeding back through the pump into the tank. When you turn on the ignition, if you get fuel pressure at the test point but when it is cranked over on the starter motor the engine doesn't fire then it probably isn't the fuel pump that is the problem. Conversely, if the engine fires, then the fuel pressure drops and causes it to misfire or stall, particularly when under load, then it is very likely to be the pump that is the problem or maybe the in-tank hoses and connections.
The arrangement of the fuel system is; the in-tank pump draws fuel through a strainer and sends it forward to one injector, from there it goes to the second injector then to a return hose which takes it to the combined fuel filter/pressure regulator canister (which is mounted to the rear bulkhead above the swing arm) and from there returns to the right hand fuel tank via a short hose to the top of the pump housing on top of the tank. This means that the pressurised part of the fuel system runs from the pump via the injectors (and test port) then back to the fuel filter. When the pump is not running the pump will allow the pressure to bleed off back into the tank up to a point at least. If the simple valve and spring in the filter isn't sealing as well as it should then it could also allow fuel to pass when the pressure drops off anyway but usually some (but not full) pressure will remain in the system for a fair while after the pump has been running. If there is crud in the tank that gets past the pump strainer then it will travel right round the system to the filter where it will be removed just before returning into the tank. While this may sound like a daft arrangement it is an extremely common one. As far as I can tell there isn't an actual one way or check valve in the system but the pressure regulating valve will largely fulfil this function.
It is more than possible to re-plumb the fuel system to allow thew fuel to be filtered before the injectors and to use a better, dedicated, pressure regulator that controls the fuel pressure much more accurately and consistently than the standard arrangement (no pressure drop at full throttle and high revs which does happen even with a brand new standard filter/regulator and new pump) plus you can even fit one or more check valves to maintain the pressure in the system if you wish, but the benefits are mainly confirmation bias related...guess how I know!
