Hi Everyone:
Still trying to figure out why an otherwise excellent engine has low oil
pressure.
Facts:
1)     Engine has 95,000 miles.
Compression is 180-192 on all cylinders.
No leaks.
2)     Oil light comes on at  1200 rpm or lower.
3)     Changed oil from unknown to 20w50
4)     Changed filter from Fram to Mann
5)     Installed mechanical oil pressure gauge.
When started from cold, oil pressure builds to over 50 PSI at 3000 rpm.
When warmed up, pressure is 30 PSI at 3000 rpm.
When warm, oil prssure is <5 at idle.

6)     Strange fact #1:
When running at around 2000 rpm steady, I watched the oil pressure go from
40+ down to around 15 in a matter of seconds. It would not build up again.

7)     Strange fact # 2:
I removed the oil pressure relief spring and plunger. The plunger was
installed wrong at some point! It was on the bottom, with the spring on top.

8)     Strange fact #3:
Even after putting the plunger in the correct way, the pressure situation
did not improve.
I even tried shimming the spring a bit to give it more tension. No
improvement at idle.

Could someone tell me what this might be?
Thanks
Rio