Well your oil pressure is definitely getting away.
We all have tried to cheat the low oil pressure thing on wore out used engines. Install new oil pump and still low.
First rule of good oil pressure, make sure all of the Rod, Main and Cam bearings are within tolerance. This is what holds the pressure up. A new oil pump after 100,000 miles is a good ideal too, can you imagine how many gallons of oil that original oil pump has pumped in 100,000 miles . . yes they wear.
Guessing the cam bearings are wore enough after 100,000 miles that they are bleeding oil and letting the pressure go low. Have been building some 360s here lately and am putting new cam bearings in all of them. After the rebuilds we are getting 25 psi at idle and 75 psi at 2000 rpm with the stock replacement Melling 72 new oil pump, new rods, new mains, polished stock crankshaft and new Melling SPD-22 camshaft with melling matching lifters.
The 360 cam bearings that came out of these engines all looked really tough, and some of them the layers started coming apart. Even if the cam bearings look ok, they are soft and can be worn beyond factory tolerance anymore.
So my guess is worn cam bearings and a stock oil pump that is not up to par. Can't believe a thing people tell you about that used engine sitting there in the guy's front yard, that it was running when pulled. If it was a good running engine it would have still been in the car. There is a reason that engine was out in the yard and up for sale. Let's say a low oil pressure issue . . .