Signs of oil pump death?

I have a 97 sport with 4.0 and AW4, 142k miles on the clock.

I think the oil pump may be preparing to die, but I'm just not sure. When cold, pressure stays between 40 and 50 psi both at idle and cruise (according to the dash gauge). As it heats up, of course the pressure drops, eventually getting down to as low as 15 psi at idle with 15w-40 in the case. Big drop from 40+ at idle. But, it does something weird. If you just sit and let it idle after it's hot and been running at least 30 minutes, the pressure gauge will 'bump' +/-5 psi randomly. Needle will stay at 20psi, then drop to 15, then back to 18 or 20....without any variance, just sitting there idling.

I've considered the pressure sending switch, but it seems to perform very consistently aside from this hot idle thing. Mechanically, the engine (main and rod bearings) should be rock-solid, as the PO maintained it immaculately so I wouldn't suspect that. For the same reasons I would assume the pump screen is clear as well.

I've heard the stock oil pumps usually don't make it past about 150k mark. Could this be the sign of a dying oil pump?

An oil pump is a positive displacement, gear-driven device. It incorporates a pressure relief valve that that regulates oil pressure by opening up and by-passing the engine if the pressure is too high (cold mornings, thick oil).
If the gears, or the gear cover is badly worn, oil pressure would be down an equal amount across the RPM range; it wouldn't fluctuate with a steady RPM.
On the other hand, the pressure relief valve could intermittently stick, causing oil pressure to fluctuate.
I'd start by installing a mechanical gauge to confirm what the electrical gauge is telling you. If you get the same readings/fluctuations, then I'd suspect sticky relief valve. If that's the case, I'd replace the pump--cheap insurance.