I have a 2002 Audi A4 V6 - two weeks ago the oil pressure light came on - I pulled over checked the oil (it was OK) started the car back up and the light was gone. this has happened 5 or 6 times in the last two weeks. I either turn the engine off and restart and it’s gone or it comes on, chimes three times, and then goes off. no real pattern except the car needs to be running for at last 20 minutes and it happens between 20 mph and 40 mph never at high speed or at idle. Any thoughts?
My thoughts at this point are as follows:
Don’t continue to drive the car until a qualified mechanic has used a gauge to measure the actual oil pressure.
We need to know the typical conditions under which this car is driven (mostly local/short-trip, mostly highway, a mixture of the two).
We need to know how often the oil is changed, in terms of both odometer mileage and elapsed time.
How many miles are on the odometer?
The problem might be caused by a faulty oil pressure sending unit/switch.
https://www.rockauto.com/catalog/moreinfo.php?pk=3149904&cc=1379594&jnid=424&jpid=0
These are usually located on the side of the engine near the oil filter.
Tester
Concur with the others, hopefully it is the oil pressure sensor (termed the “sending unit”) is faulty; or , less hopeful, it’s something potentially expensive. Like oil sludge blocking some of the engine’s internal oil paths, or internal oil leaks due to engine bearing wear.
+1 for the concur.
134,000 miles; in recent years mostly local/short trips. Again in the last couple of years oil is changed every 6 months - only doing less that 6,000 miles per year.
I, for one, would also like to know how often the oil is CHECKED.
A mechanical oil pressure guage check is in order. The likely case is a faulty switch but diagnosis is needed before throwing parts at it. Cross your fingers for electrical issue 
Hopefully this is an oil pressure sending unit problem. With the oil being changed every 6 months and 3k miles a more serious concern would be an oil sludging or coking issue.
The low miles per time driven can contribute greatly to a sludging or coking problem. Knock on wood that it’s the pressure sender.