Low oil pressure light at idle

Hi.

For sometime now, I have been experiencing some issues with my car whereby after driving more than a mile and coming to a stop (idle) my engine light will start flashing on the dashboard. See steps below that re-creates the issue:

1. Cold engine start - Okay!
2. Driving less than a mile - Okay!
3. Driving more than a mile and coming to a stop (idle) - Engine oil pressure start flashing on the dashboard.
4. If the car is allowed to rest a few hours, the issue disappears, but re-appears again when driving a long distance.

My local mechanic first thought it was the oil pressure switch so we changed that, but still no luck. He removed the oil sump and found some "thick" oil residue in the pan and some of it where clogging the oil screen. He cleaned it, changed the oil and drove a few miles, but issue still remained. So he finally said it "might" be the oil pump so I bought a new one, replaced it, but issue still remains.

Do note that the engine does not make any weird sound or show any signs of engine performance issues. I am really out of options here.

Will you be able to help me in identifying what could be the cause of the issue please?