3.1 GM, Idle drops, oil pressure drops

[P99]

Good Day, hope you guys can help.
1991 Rodeo, 3.1 rebuilt "professionally" 2,000 Miles ago.
Ran fine for about 1,500 miles, good power, oil pressure Ok, no problems.
Now not so good.
Start up, no problem, starts right up, idles at about 1100, oil press 4 -6x100KPA at idle
First 10 Mins of driving, warms up fine, idle drops to about 750, oil press 2 - 3x100Kpa at idle, idles fine, power is good, all normal.
After about 15 mins of driving, idle at about 700 and oil pressure at 2x100Kpa, all seems normal.
After about 20Mins of driving and come to stop light, rpm drops to about 650, oil press drops to about 1x100KPA, oil light comes on, headllights flicker or go out, buzzing sound coming from drivers side of engine compartment. Engine is running smoothly, doesn't stall.
Turn off engine and restart
Starts fine, oil press normal, at idle revs are about 750, all ok. Give it a bit of gas and let off, oil press drops to back to 1x100Kpa, idle drops down to about 650. Give it a bit of gas and raise rpm up to 700 and all ok. Drives no problem, power good, all normal, come to a stop and idle drops, oil pressure drops, lights go out.

1st time back at the shop that rebuilt the motor. "Did some adjustments, all is good you should have no problems now". 20mins of driving same problem
2nd time back at the shop that rebuilt the motor. "You need a new ICA Valve, we installed a used one that checks out fine, you will have no problems, all Ok, that will be $140.00" 20 mins later driving home, same problem.
3rd day back at the shop is tomorrow!

Help, any ideas.

Any suggestions that you may have on what is causing this problem would be most appreciated. Would be nice to have a bit of an idea what it could be before I go back for the third time and the shop says you need this or that.

Thanks in advance, P99