It's very frustrating, isn't it?
I tell customers that I'm as patient as they are. If I need to keep your car for a week, I will. We'll use it (with your ok, of course) as a shop vehicle, using it to drive to the bank, pick up lunch or parts, run shop errands, etc. Doing that, it should be possible to duplicate the concern. A dealership technician probably doesn't have that kind of freedom. And an experienced tech can probably tell by feeling the concern if the problem is related to fuel supply, ignition, fuel delivery, engine sensor, or even electrical.
As far as testing, a fuel pressure gauge is no longer the only way to test a fuel pump. We regularly test the electrical condition of the pump using a "digital storage oscilloscope" and a low-amp probe. This gives lots of information to someone who knows how to interpret it. The fault could also be with the ignition system (plug wires, failing coil), a failing sensor (throttle position comes to mind), an electrical component (like a fuel pump relay), perhaps even a fuel injector failing only when hot. We also use a scan tool that can record all engine data like a movie that we then look at frame-by-frame back at the shop to help us find the fault.
Needless to say, this takes time and money. But then so does working down the list of "likely causes" until you find the right one.