Audi A6 99 2.8 Quattro - Low Oil Pressure

Hi, Re posting the same old problem after few months.
Car: Audi A6 99 2.8L Quattro. 113Kmiles

History: Started having low oil pressure problem since Apr 2011
1) Checked oil pressure and they all checked out fine as per my mechanic
2) Changed oil pressure sensor or sender and still same issue
3) No noise when light comes on and car runs with same power, no other abnormalities
4) Have been driving like this almost 6months without any issues but annoying beep without any damage so its not real pressure issue.

After so much observation below is the pattern, after a bit of driving and when car reaches normal working temp around 150F to 200F, low oil pressure warning starts only at 1800RPM.
Took car on plain empty road and did a test.
1) Drive below 1800RPM - No light
2) Drive above 2000RPM - No light
3) Drive app around 1800RPM for morethan 2Secs - BEEP BEEP and light comes on, if I press acc or remove acce light vanishes.(More over its city driving where car runs at 1800RPM)

I know that oil pump runs faster if engine revs fast and slow when env revs slow.
Million Dollar question -- Is it oil pump? Or something else like engine rev sensor or anything gurus here can think off?

I am very determined to fix the issue without throwing money at it guessing as this is my winter car and love to keep it for short drives.

Thanks for reading.