Fuel Pump Clue requested

I have a Zenith CH-300 with a Lyc 0-320. It has an engine mounted fuel
pump, and an electric aux pump. I also have a fuel pressure gauge, and
a low fuel pressure warning light. Today, for the first time, when I
did my pre-flight run-up, the low fuel pressure light came on. When I
looked at the fuel pressure gauge, I was showing 2 pounds, or maybe
just a touch less. When I turned on the boost pump, the pressure went
right back up to 6 pounds or so.
In flight, with the boost pump off, the pressure shows about 3 or 4
psi. That is with partial throttle, and an rpm of maybe 2300 to 2400.
If I go to full throttle, the pressure will drop to around 2 psi, and
the low fuel pressure light will come on. The engine never gives any
indication of being starved for fuel. But something is changing. The
key question I have here, is how do fuel pumps wear out. Is a
reduction in delivered pressure or volume symptomatic of the pump
nearing the end of it's service life? Or maybe we have a partially
blocked check valve that is leaking a bit? Does the spring inside get
weak with time?
And, one last question, where is a good (reasonable price) source for
a pump? I have looked at Trade-A-Plane, and sticker shock is still in
effect.

Thanks.

Zenith CH-300 Driver.