[MT-2500]

60/66 lbs are specs.
59 is to low.
Make sure your gauge is good.
On the fuel pressure test you need more testing.
Fuel pressure specs are 60/66 on your engine.
Key on pressure is good.
But we need the engine cranking pressure.
Your engine running pressure is dropping below specs.

Engine running should range about 61-62 full vacuums no load or idle.
A throttle snap or a load on engine no vacuum the pressure should go to 65-66 range.
Steady driving down the road or light throttle it should range 63 -64.

But you need to get a fuel pressure gauge taped to the outside of windshield and drive it 30 minutes to a hr and watch the fuel pressure after the engine and fuel pump has warmed up good.
And also watch the fuel pressure when it acts up.
Like catching it in the act of acting up.

From what you have checked your fuel pressure is dropping down after engine is started.
But that does not always say the fuel pump is bad.
The fuel pressure regulator controls pressure as long as the fuel pump has pressure to control.
You need to check the direct fuel pressure at the fuel filter to check pump.
It should put out 75-85 lbs of direct pressure to be good.
Also a pinch the return line of will give you a fuel pump or fuel pressure regulator check. Which should also be done after fuel pump has been run 30 minutes or more.
So before you replace the fuel pump do the full testing on it.
You could have a fuel pressure regulator problem or other problems besides the pump.
MT