2001 Volkswagen golf 1.4 petrol

After car is running for around 20 mins and when car is revved up over around 2000 rpm the red oil light makes a beep sound and comes on flashing intermittently for a few seconds before going off. Sometimes I have to switch off the car and when I turn it back on the light is off. Usually only comes on if I’m driving for a substantial journey usually over 20 minutes. Oil is at the max on the dipstick too. Cheers.