Intermittent Oil Pressure

He said he put a new oil pump in it.

Things I would do. Like John said in post 2. Put a GOOD mechanical gauge on it. If you see the pressure loss THERE, get her back out again.

I would from there, inspect the camshaft gear VERY closely. They have been known to fail. Look at the oil pump gear too. There was a rash of oil pump gear failures with the slant six for a time. If both those look good, "MY" thought is, there is a blockage in the system somewhere....probably some sludgy crap hung up in an oil galley. Unscrew the oil pickup. Inspect it very closely. Sometimes they can get sludge buildup under the shield and you cannot see it. Make sure the tube is unobstructed. Also make sure the passage in the block to the pump is clean and clear.

I would remove the oil galley plug directly behind the cam gear and apply shop air pressure to see if something gets dislodged. This if course with the oil pan off and the oil pump removed. I say go in at the oil galley behind the cam gear because this will be applying pressure backwards and is more likely to dislodge "something" if anything is there. You'll also not want to be standing in the area where the oil pump mounts, rather, I would hold a gloved hand holding a shop rag over that area to see if you catch something on the way out.

Next, I would remove the rocker shaft and rockers. Find the oil feed hole in the head. On the slant six, the hole for the oil feed is in the very last rocker shaft pedestal, that would be closest to the firewall. Apply shop pressure there. As mentioned, this will be applying pressure backwards through the system and possibly dislodge any obstructions.

Beyond that, You can also run engine cleaning brushes through the oil galleys. "I" would much rather do that with the engine torn completely down at that point though, so I could rewash the block as if it has just come from the machine shop.

The father down you go and more you do, you'll find "whatever" it is eventually.

Now, before you go runnin back out to yank it back out.......you HAVE tried another filter, right?